Halachos of “v’sain tal u’matar l’vracha”:
1. If you remember before “u’varech shenoseinu” in Birchas hashanim: Say it where you are and continue from there.
2. If you already started “uvarech shenoseinu” but didn’t say Hashem’s Name yet: GO BACK to “v’sein tal umatar l’vracha” and continue with the words that come after that.
3. Once you said Hashem’s Name, say it in Shema Koleinu.
4. If you remember right after Hashem’s Name, it is better not to say lamdeini chukecha and go back, since you have the option of saying it in shema koleinu (unlike mashiv haruach which CANNNOT be said in shema koleinu)
5. If you forget in Shema koleinu, but you did not say Hashem’s Name yet, you should say it then. I think that in this case, you should GO BACK and continue with “ki Atah shomeiah”. The MB does not specify clearly but it is mashma from other things he says (the general rule being that you have to have a chasima m’ain chasima). If anyone knows where it is stated clearly, please let me know. It could be that he just considers it to be obvious from s”k 19.
6. If he had just said Hashem’s Name at the end of shema koleinu but not gone further, he should say “lamdeini chukecha” and continue with “ki Atah Shomeiah”.
7. If he said at least one word after Hashem’s Name but did not start Retzi yet, he should say it in between the two brachos of shema koleinu and retzei (since he is still considered to be in the middle of shema koleinu) and continue with Retzei.
8. If he started Retzei but did not yet finish the “yihiyu l’ratzon” at the end of Elokei netzor, he should go back to “Bareich Aleinu” (I assume that if he said Hashem’s Name at the end of the “yihiyu l’ratzon”, he should finish the passuk first).
9. If he finished the “yihiyu l’ratzon at the end of “Elokai netzor” he should go back to the beginning of Shemona Esrei.
10. If he finished the “yihiyu l’ratzon” at the end of Elokei netzor but it is ‘toch k’dei dibbur” (the amount of time it takes to say “Shalom aleichem Rabi’), he should still go back to “bareich aleinu”.
11. These halachos stay the same whether or not he also says the “y’hiyu l’ratzon” before Elokei netzor.
If I made any more taus sofrim’s or mistakes, please feel free to let me know. Thanks!
ps: sources to follow, bli neder.