I would appreciate a response to:  I forgot to say mincha before shkia. Once bein hashmashos arrives, is it too late to say mincha? do I wait to tzais, then say maariv and then do tashlumin mincha?  Thank you.

ב"ה Shalom According to the Mishna Brura one should Daven mincha before Shki'ah, however after the fact, person can Daven mincha after Sh'kiah. (Mishna brura 233:18) until a certain time. (There are issues of when is Shkia'ah and the major difference of opinion between the calculation of Rabbenu Tam and the Ge'onim, Which I will not get into here.) However, how much time after Mincha one may Daven Ma'ariv in a non-ideal situation is dependent upon where you live. The Minhag in Eretz Yisrael is that one may lenient to Daven mincha during Bein Hashmashot (according to the opinion of the Ge'onim ) which is up to 13 and one half minutes after shkia'ah. (Piskei Teshuvot 233:8) However, in the United States, according to the Halacha Bulletin of the Kaf-K ," Halachically Speaking", compiled by Rabbi Moishe Dovid Lebovits, though some poskim say one can Daven Mincha up to twenty minutes Shhkia'ah, one should Daven only up to 9 minutes after Shkia'ah (Opinion of Harav Moshe Feinstein zt”l as related by Harav Yisroel Belsky zt"l, Refer to Igros Moshe O.C. 1:24. All the best