Changes in the Amidah applicable to the ten days of repentance:
In Shemoneh Esrei of Aseres Yimei Teshuvah the third blessing concludes with the words “Hamelech Hakadosh” in place of “Hakel Hakadosh”. This begins from the first night of Rosh Hashanah until the end of the ten days of repentance.
Forgot to say Hamelech Hakadosh: If one did not say Hamelech Hakadosh, or is in doubt if he said it, then if he already began the next blessing, he must repeat Shemoneh Esrei. [This applies even by Maariv of the first night of Rosh Hashanah. This applies even if he remembered within Kdei Dibbur of “Shalom Alecha Rebbe”.]
Remembered prior to beginning the next blessing: If he has not yet begun the next blessing, then if he has not passed the amount of time it takes to say Shalom to his Rebbe, [which is the amount of time it takes to say the three words of “Shalom Alecha Rebbe”] then he may say Hamelech Hakadosh and continue. If however he remembered only after this amount of time already passed, then he must repeat from the beginning of Shemoneh Esrei. This applies even if he did not yet begin the next blessings, and certainly applies if he already began the next blessing.
Mieiyn Sheva: On Shabbos Teshuvah, as well as when Rosh Hashanah and Yom Kippur fall on Shabbos, the Chazan recites in the blessing of Mieiyn Sheva recited [after Shemoneh Esrei of Maariv] on Friday night, Hamelech Hakadosh in place of Hakeil Hakadosh. [Likewise when the congregation customarily recites the paragraph of Magen Avos they say the words Hamelech Hakadosh in place of Hakeil Hakadosh.] The Chazan however concludes only with the blessing of Shabbos [i.e. Mikadeish Hashabbos] even if Rosh Hashanah or Yom Kippur falls on Shabbos.
If one is in doubt as to whether the time of Kdei Dibbur has passed what is he to do?
If one fixed himself and recited Hamelech Hakadosh but is in doubt as to whether the time of saying “Shalom Alecha Rebbe” had already passed, then he is to continue with Shemoneh Esrei.
If one remembered after saying the word Hakeil, prior to the word Hakadosh, but the time of Kdei Dibbur has passed, what is he to do?
He fulfills his obligation and is to continue with Shemoneh Esrei.
If the Chazan forgot to say Hamelech Hakadosh what is he to do?
He is required to repeat from the beginning of Shemoneh Esrei, just as is the law for an individual. In such a case Kedusha is to be repeated.
If one remembered by Zachreinu or Mi Chamocha that he needs to conclude with Hamelech Hakadosh and is now in doubt as to how he concluded, does he have to repeat Shemoneh Esrei?
Some Poskim rule that he is not to repeat Shemoneh Esrei. Nevertheless if he remembered in middle of Shemoneh Esrei he is to think in his mind from the beginning of Shemoneh Esrei until his current area and then continue verbally from where he left off. [Seemingly however according to Admur and other Poskim it is best to repeat the prayer as a Nedava.]
Hamelech Hamishpat: All the above is only with regards to Hamelech Hakadosh, however regarding Hamelech Hamishpat we do not assume that one recited it even if he remembered to say Zachreinu in the beginning of Shemoneh Esrei. This applies even if he remembered to say Hamelech Hakadosh.
If after Shemoneh Esrei one has doubt as to whether he said Hamelech Hakadosh, does he have to repeat Shemoneh Esrei?
If the doubt entered his mind immediately after Shemoneh Esrei, he is to repeat the prayer. This applies even if he knows that at the beginning of Shemoneh Esrei he intended to recite Hamelech Hakadosh. If however the doubt entered his mind a while after finishing Shemoneh Esrei then if at the beginning of Shemoneh Esrei he knew that he needs to recite Hamelech Hakadosh, he is not required to repeat Shemoneh Esrei.
If in Meiyn Sheva the Chazan said Hakeil Hakadosh instead of Hamelech Hakadosh must he repeat the Meiyn Sheva?
This matter is disputed amongst Poskim. Some Poskim rule if he completed the blessing, then the blessing must be repeated from the beginning. Others rule it is not to be repeated. Practically the Chazan is not to repeat the blessing. Nevertheless if the Chazan remembers prior to saying G-d’s name in the blessing of Mikadeish Hashabbos then he is to go back and say it.
 582/1-2; Michaber 582/1
 The reason: The reason for this is because during Aseres Yimei Teshuvah G-d shows his Kingship to the world through judging all his creations. [Admur 582/1; Levush ]
 The reason we are stringent by a doubt: The reason for this is because a person has a Chazaka that he recites his normal tongue. [Beis Yosef; Levush; M”B 582/3]
The reason: As the first three blessings are all considered like one blessing. [Admur ibid; Levush]
Other Opinions: Many Rishonim [Ravaya; Igur; Maor; Raavad] hold that from the letter of the law if one recited Hakeil Hakadosh he is not required to repeat Shemoneh Esrei. Some Poskim conclude that although we do not hold their opinion nevertheless it is proper to join their opinion to other cases of doubt and not require one to repeat Shemoneh Esrei. [See Zechor Leavraham 1 “Shin” and 3 Mareches Reish 138; Kaf Hachaim 582/6]
If one recited Hamalech Hakadosh 90x:
Even if one recited “Baruch Ata Hashem Hamelech Hakadosh” 90x to himself prior to Shemoneh Esrei he still must repeat Shemoneh Esrei in case of doubt. The reason for this is because although in a case of doubt we assume one has said whatever statement he commonly says, which is determined by if he said the statement 90x, nevertheless here since one was never allowed to say Hashem’s name in the blessing 90x it is therefore not considered as if he commonly has said the blessing of Hamelech Hakadosh, as when he now recites Hashem’s name in the blessing he returns to his common conclusion of Ha-kel Hakadosh. [582/3; M”A 582/1; Shalmei Tzibur p. 308; Shaareiy Teshuvah 582/1; Elya Raba 582/4; Chayeh Adam 24/10; M”B 582/3; Kaf Hachaim 582/3]
Other Opinions: Some Poskim rule that if one had recited “Baruch Ata Hashem Hamelech Hakadosh” for a total of 90x prior to the current Shemoneh Esrei then he is not required to repeat if he does not remember whether he said it. [Peri Chadash; Birkeiy Yosef 582/3]; M”E 582/7 rules that since there is a doubt one is to repeat the Shemoneh Esrei as a Nedava.
Initially: One is to avoid initially reciting Hamelech Hakadosh 90x in order to avoid the above dispute. [Kaf Hachaim ibid]
 Implication of Admur ibid and all the other Poskim which have not differentiated between the nights of Rosh Hashanah and the other days. So rules explicitly: Mayim Chaim 22; Kitzur SHU”A 129 in Peas Hashulchan; M”B in Shaar Hatziyon 582/4; Kaf Hachaim 582/3; Alef Hamagen 582/19; Sdei Chemed Mareches Rosh Hashanah 3; Shoel Umeishiv Rivia 3/125; Beis Halevi 1/42; Hagahos Harashash 582; Chaver Ben Chaim 1/33;
Other Opinions: Some Poskim rule that on the first night of Rosh Hashanah one does not repeat Shemoneh Esrei. [Chayeh Adam 24/10; Binyan Shlomo 2/46; Igros Moshe 1/170] The following is the background of this opinion: The Chayeh Adam 24/10 writes in the name of the Gaon Rav Avli Pasvellor that the first night of Rosh Hashanah is similar to the night of Rosh Chodesh on which the ruling is that one who forgot Yaleh Veyavo does not repeat Shemoneh Esrei. Thus if on the night of Rosh Hashanah one forgot to say Hamelech Hakadosh he does not repeat Shemoneh Esrei. The reason for this similarity is because the judgment of Rosh Hashanah is dependent on when is Rosh Chodesh, and since Rosh Chodesh is not sanctified at night, similarly the judgment does not begin at night. [Binyan Shlomo ibid; Igros Moshe ibid] All the above Poskim however negate this opinion. However the Binyan Shlomo and Igros Moshe defend it. Regarding the second night of Rosh Hashanah the Binyan Shlomo ibid leaves this matter in question, although the Igros Moshe ibid rules that one must repeat Shemoneh Esrei.
 Mateh Yehuda; Shaareiy Teshuvah 582/2; M”E 582/9; M”B 582/7; Kaf Hachaim 582/10; Misgeres Hashulchan 129/3 that so is implied from Admur
 Admur 206/3; Kneses Hagedola 206; Olas Tamid 206/3; M”A 206/4; Elya Raba 206/5; Chesed Lealafim 206/3; Ben Ish Chaiy Balak 3; Kaf Hachaim 206/15; 582/9
Other Opinions: Some Poskim rule that it contains four words “Shalom Aleichem Rebbe Mori”. [Beis Yosef in name of Shivlei Haleket; Levush; Taz 206/3; P”M 487]
 Admur 582/2; This applies even if he originally intended to concluded Hakeil Hakadosh due to forgetting that it is Aseres Yimei Teshuvah. [M”A 582/2; P”M 582 A”A” 2; Biur Hagr”a M”E 582/9; M”B 582/8; Kaf Hachaim 582/11]
 Admur 582/1
The reason he cannot just repeat the blessing of Ata Kadosh: As the first three blessings are all viewed as one long blessing. [ibid]
 Admur 582/1; Michaber 582/2
Other Opinions: Some Poskim rule that so long as one has not begun the first word of the next blessing he may say Hamelech Hakadosh. [Kaf Hachaim 582/9; Ashel Avraham Butchach 582 leaves this matter in question] Practically the Poskim negate this opinion. [Yabia Omer 7/51] See Piskeiy Teshuvos 582 footnote 15.
 Admur 582/4; Michaber 582/3 [regarding Shabbos Teshuvah]; Rama ibid [adds Rosh Hashanah and Yom Kippur]
 Seemingly this latter statement is only relevant when Rosh Hashanah or Yom Kippur falls on Shabbos, and hence the novelty is that one does not add Rosh Hashanah or Yom Kippur in the concluding blessing. However on Shabbos Teshuvah there is no reason to think otherwise, that anything else should be added in the concluding blessing other than Shabbos, Upashut.
The reason: The reason why we do not mention Yom Tov [Rosh Hashanah or Yom Kippur] within this blessing of Meiyn Sheva is because its entire reason of being said only relates to Shabbos, as on a regular Yom Tov Meiyn Sheva is not recited. It is recited on Shabbos not due to the holiness of the day [which would thus warrant it to be recited also on Yom Tov] but rather due to Mazikin [damaging forces]. It is thus not similar to the law that requires Shabbos to be mentioned in Niela of Yom Kippur, that although Neilah is never recited on Shabbos and is only recited due to Yom Kippur, nevertheless one is required to mention Shabbos in the Shemoneh Esrei. This is because Yom Kippur obligates four prayers to be recited, which is Shacharis, Musaf, Mincha, and Neilah, and thus one must mention Shabbos in each prayer. However here the blessing of Meiyn Sheva was not instituted due to the holiness of the day [as was Neilah of Yom Kippur] but simply due to the Mazikin. [269/14]
 Ashel Avraham Butchach 582
 Daas Torah in name of Daas Kedoshim brought in Piskeiy Teshuvos 582/5
 Zekan Ahron 206; Shalmei Tzibur p. 306; Zechor Leavraham “Shin”; Shaareiy Teshuvah 582/1; M”E 582/8; Ben Ish Chaiy Netzavim 18; Kaf Hachaim 582/5; Or Letziyon 1/13
Other Opinions: Some Poskim rule that the Chazan is not required to repeat from the beginning of Shemoneh Esrei but is rather to begin from Veata Kadosh. The reason for this is because we do not apply the rule that the first three blessings are considered like one blessing with regards to the Chazan’s repetition. [Yabia Omer 1/8; 7/51 based on Meiri Brachos 34a; However see Or Letziyon 1/13 that disputed his ruling]
 Shaareiy Teshuvah ibid; Zechor Leavraham ibid
 Within the Amidah prayer of Rosh Hashanah the prayer of ובכן תן פחדך is added in the third blessing.
 Mayim Chaim 22; Chayeh Adam 28/8; M”B 582/4; Kaf Hachaim 582/6; Beis Halevi 1/72; Galya Misechta 1; Alef Hamagen 582/19
Other opinions: The Sdei Chemed [Mareches Rosh Hashanah] rules that one must repeat Shemoneh Esrei even if he recited Vechein Ten Pachdecha. This is proven from the fact that the earlier Poskim did not differentiate in this matter. [See Alef Hamagen ibid]
 The reason: As we assume since he remembered to add the additional prayer of Vechein Ten Pachdecha certainly he remembered to conclude with Hamelech Hakadosh which is the conclusion of that blessing.[Chayeh Adam ibid] Alternatively there is a true doubt involved in this case as we cannot assume he said the wrong Nussach being he said Vechein Ten Pachdecha, hence we apply Safek Brachos Lihakel and he does not repeat Shemoneh Esrei. It cannot be repeated as a Nedava as we do not Daven a Nedava on Yom Tov. [Galya Misechta ibid]
 Ashel Avraham Butchach 582
 The reason: As perhaps since he remembered to add the additional prayer of Mi Chamocha he also remembered to conclude with Hamelech Hakadosh, and hence the matter is a true doubt. [ibid]
 As according to some Poskim [P”M 62 M”Z 1] thinking the prayer is valid in a case of “Ones”. This then creates a Safek Sfeka which exempts one from needing to repeat Shemoneh Esrei even as a Nedava. [Ashel Avraham ibid]
 As the Peri Chadash 62 and 94 rules that thinking the prayer is invalid even in a case of Ones and he hence in this case there is only one Safek. So is also implied from Admur 62/3 that one does not fulfill his obligation at all through thinking the words, unlike the P”M ibid.
 Ashel Avraham ibid; Thus according to Admur it would be proper to repeat Shemoneh Esrei as a Nedava while according to the Michaber one would be required to repeat Shemoneh Esrei.
 Or Li 111; Yabia Omer 7/51-3
 M”B 114/38 [regarding Mashiv Haruach and Yaaleh Veayavo]; Piskeiy Teshuvos 582/2
 See Kaf Hachaim 582/18; Mateh Efraim 582/5; Alef Lamateh 582/1; Alef Hamagen 582/17; Piskeiy Teshuvos 582/6
 M”B 582/10
 Kneses Hagedola 582/5; Elya Raba 582/2; Mateh Yehuda; Mateh Efraim 582/5; Shalmei Chagiga in name of Mahariy Iash [brought in Alef Lamateh ibid]; Machazik Bracha 582/3; Shalmei Tzibur 197; Zechor Leavraham 3/143; Shaareiy Teshuvah 582/3; Ben Ish Chaiy Netzavim 18 The reason: The Chida in Machazik Bracha explains that according to Kabala there is a mystical reason behind reciting Meiyn Sheva and hence it must be repeated if not recited properly. [Kaf Hachaim ibid]
 If however he did not yet complete the blessing then he returns to the words Hamelech Hakadosh and reads from there and onwards. [ibid]
 Peri Chadash [brought in Kaf Hachaim ibid]; Gan Melech 149; Siddur Derech Hachaim; Peri Megadim 582 A”A 2 ; Nishmas Adam; Yeshuos Yaakov; Pischeiy Teshuvah in name of Divrei David 55; Alef Hamagen 582/17; Kitzur SHU”A 129/4
The reason: As the entire blessing of Meiyn Sheva is only recited use to Sakana, and hence we do not require its repetition if a mistake was made. [Peri Chadash ibid]
 Likkutei Mahrich; Luach Eretz Yisrael; Piskeiy Teshuvos 582/6; The Kaf Hachaim ibid concludes that one who does not repeat the blessing is not to be protested, although it seems he leans like the stringent opinion viewed by the Chida, based on Kabala. See Yabia Omer 2/29.
 M”B ibid; M”E ibid; See Peri Chadash ibid that “although if he is still within Kdei Dibbur we protest him and make him return”. Vetzaruch Iyun as to which part of the prayer he is referring to Kdei Dibbur of? The words Hakeil Hakadosh or the concluding blessing.