Mashiv Haruach Umorid Hageshem:
What is the request of Mashiv Haruach? The purpose of the prayer of Mashiv Haruach is not to request for rain. This request only begins in the month of Cheshvon with the request of Vesein Tal Umatar Levracha. It is rather an introductory prayer of appeasement to Hashem prior to beginning the request, as prior to asking for rain at its proper time we appease Hashem through mentioning His power of descending rain.
Where do we say it? The statement of Maashiv Haruach is recited in the blessing of Mechayeh Hameisim. The reason for this is because rain is considered similar to the resurrection, as just like the resurrection gives life to the world, similarly rain gives life to the world.
When does one begin to say it? Mashiv Haruach Umorid Hageshem is recited beginning from Musaf of Shemini Atzeres. On Shemini Atzeres prior to the silent Musaf the Chazan announces Mashiv Haruach Umorid Hageshem and it is then recited in the Musaf prayer and every prayer thereafter.
Until when is it to be said? Mashiv Haruach Umorid Hageshem is recited until, but not including, Musaf of the 1st day of Pesach. Those which Daven the Nusach of Arizal/Sefarad, omit Mashiv Haruach and begin reciting Morid Hatal from the silent prayer of Musaf of the 1st day of Pesach. The Chazan thus announces prior to the silent prayer of Musaf “Morid Hatal”. Those however which follow Nusach Ashkenaz still say Mashiv Haruch Umorid Hageshem by the silent Musaf of Pesach and only begin to omit with the repetition of the Chazan by Musaf and all prayers thereafter.
If one forgot to say it:
- Nusach Arizal/Sefarad: Those which Daven Nusach Arizal or Sefarad do not go back in the prayer if they accidently omitted Mashiv Haruach, as in any event they are accustomed to say Morid Hatal in the summer days that Mashiv Haruach is not said, and this mentioning suffices in place of Mashiv Harucah. Thus even if one did not yet finish the blessing of Mechayeh Meisim he is not required to go back.
- Nusach Ashkenaz: Those which Daven Nusach Ashkenaz, if they omitted Mashiv Haruach, then if they already concluded the blessing and began saying the next blessing [even the first word of “Ata”] they must return to the beginning of the prayer, irrelevant to where in the prayer they remembered of their omission. Similarly if they remembered only after concluding the entire prayer they must repeat the Davening. If one remembered prior to saying Hashem’s name in the blessing of Mechayeh Meisim, then he is to say it in the place that he remembers and continue from where he left off. [If he already said Hashem’s name then he is to finish the blessing and say it prior to beginning the words of the blessing of Ata Kadosh. The same applies] if he only remembered after concluding the blessing of Mechayeh Meisim but before beginning the next blessing, then he is to say it there. If however he already began even the first word of the next blessing he must return to the beginning of the prayer, as explained above.
If one is in doubt if he said it: This law has no ramifications to those which follow Nusach Arizal/Sefarad as explained above, hence the following ruling only applies to those which Daven Nusach Ashkenaz: Those which Daven Nusach Ashkenaz which are in doubt if they mentioned Mashiv Haruach are to consider it as if they omitted it for the first 30 days after Shemini Atzeres [until 22/23 Kisleiv], and hence follow the ruling mentioned above discussing what to do upon remembering that it was omitted.
 114/1; Tur 114
 Meaning he did not yet say Hashem’s name in the concluding blessing.
 As even if they certainly forgot to say it there is no need to return and say it.
 This depends on if we count Shemini Atzeres as the 1st day or not. Vezatrcuh Iyun.
 Prior to the conclusion of the 30 days one is not to repeat the beginning of the blessing of Michayeh Meism until … Morid Hageshem for a total of 90x in order to make for oneself a status quo that he has now regulated himself to say it and hence no longer needs to go back in case of doubt, as there are opinions which argue on this ruling and hence one ends up entering himself into a dispute if he does not remember if he said it prior to the 30 days concluding.