From Pesach until Succos some people say Morid HaTal and some don't say anything. On Shmini Atzeres we all begin to say Mashiv UMorid HaGeshem. If you forget to say it and already finished the Bracha, it depends on what you said in its place. If you said Morid HaTal then you need not go back, and what you said is sufficient. However if you did not say anything then you need to go back to the beginning of Shmoneh Esrei. (Shulchan Aruch OC 114:5)
The Biur Halacha brings that Rebbi Akiva Eiger says that on Friday night if skipped Mashiv HaRuach there is no reason to go back. He explains that on Friday night if you say a Bracha Mei'Ein Sheva, one single Bracha for Shmoneh Esrei encompassing all the seven brachos that we say on Shabbos, you are yotzei Shmoneh Esrei. In this Bracha we do not say Mashiv HaRuach. From here we see that you can be yotzei on Friday night without saying anything. However L'Halacha he remains Tzarich Iyun.
Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.
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