Said Borei Nefashos instead of Mieyn Shalosh/Al Hamichya/Al Hapeiros/Al Hagafen:[1]

One who said the after blessing of Borei Nefashos Rabos in place of the after blessing of Meiyn Shalosh [i.e. Al Hamichyah/Al Hapeiros/Al Hagfen] does not fulfil his obligation and is hence obligated to repeat the blessing of Borei Nefashos. [It certainly does not fulfill the blessing of Birchas Hamazon. One is to say Baruch Sheim Kevod Malchuso Leolam Vaed on the blessing said in vain, and then recite the correct after blessing.[2] Accordingly, whenever in doubt as to whether an after blessing of a food is Meiyn Shalosh or Borei Nefashos, it does not help to say the blessing of Borei Nefashos, and it is forbidden to do so.[3] Likewise, if one does not know the dialect of Meiyn Shalosh, it does not help to say the blessing of Borei Nefashos, and it is forbidden to do so. One is to publicize this matter, as many are ignorant about this law.[4] It is therefore incumbent on each person to know the blessing of Meiyn Shalosh by heart.[5]]

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[1] Seder 1/18; Luach 1/18; Levush 208/13; See P”M 208 A”A 26 in explanation of Levush; See Beis Yosef 202; Ketzos Hashulchan 60/6; M”B 202/42 and 55; 208/62; See Piskeiy Teshuvos 207/2

Other opinions: Some Poskim rule that Borei Nefashos is a Bracha Koleles just like Shehakol and is Bedieved a valid after blessing for any food, even bread. [Kneses Hagedola in M”A 202/26; Kaf Hachaim 202/79; 204/38; 208/38; Eretz Tzevi 1/29 [however he later contradicts himself in 2/389]; Igros Moshe O.C. 1/74; Beir Moshe 3/35 in name of Rav Akiva Eiger; See Yabia Omer 1/12; Piskeiy Teshuvos ibid footnote 6] Accoridngly, if one already ate a Shiur of bread/Mezonos/7 Minim and does not have a Siddur to Bentch, he is to say Borei Nefashos. [Igros Moshe ibid]

The law of Tziruf and Maaseh Kdeira: To note that despite the above ruling, we rule that a Meiyn Shalosh food can join a Borei Nefashos food for a Shiur Kezayis and have Borei Nefashos recited on it. [Seder 8/7; Luach 2/8] See also Admur Seder 1/8 and Luach 1/7 that in the case of dispute of Maaseh Kdeira one is to say a Borei Nefashos, even though there is dispute as to its after blessing, if it is Borei Nefashos or Al Hamichyeh. Vetzrauch Iyun.

[2] Piskeiy Teshuvos 208/1

[3] Machatzis Hashekel 202/26; P”M 202 A”A 26; M”B 202/42 in name of Peri Megadim; M”B 202/55; 208/62; Piskeiy Teshuvos 208/1; See however Kneses Hagedola in M”A 202/26 who says to say Borei Nefashos. M”A ibid concludes with Tzaruch Iyuin due to above issue.

[4] Likkutei Maharich on Seder Birchas Hanehnin

[5] Kitzur SHU”A 51/8