We begin saying Mashiv haruach umorid hageshem during the Musaf prayer on Shemini Atzeret. Mashiv ha-Ruach is added in all the Shemoneh Esreh prayers up to Musaf on the first day of Passover when the chazan says Morid ha-Tal.

If one forgot to say Mashiv ha-Ruach in the proper place (i.e., in the beginning of the paragraph that begins with the words mechalkel chayim) but remembered before saying the blessing at the end of the paragraph (Mechayeh ha-Metim), he should say Mashiv ha-Ruach and then the berachah. If he remembered after having made the berachah (of Mechayeh ha-Metim) but before having started Attah Kadosh, he should say Mashiv ha-Ruach and does not have to begin Shemoneh Esreh again. If he only remembers after having begun the berachah of Attah Kadosh, he must recite Shemoneh Esreh from the beginning.

If one reached the end of Shemoneh Esreh and cannot remember whether he said Mashiv ha-Ruach or not: If thirty days have passed since Shemini Atzeret (when one started to say Mashiv ha-Ruach), we assume that he did say it. If less than thirty days have passed, we assume that he did not say it.

One who follows the custom of saying Morid ha-Tal in the summer months, who said this instead of Mashiv ha-ruach u'morid ha-geshem in the winter and only realized that he had done so after having recited the berachah of Mechayeh ha-Metim, need not start Shemoneh Esreh from the beginning (since he mentioned the tal).