The shul where I daven starts Maariv on Sat. night 10 minutes before Shabbat is over. 2 questions: 1. When saying the "ata chanatanu" in shemoneh esrei, it is still Shabbat. Does this statement still have an effect of havdalah so that when Shabbat is over one is allowed to do melacha, or must I also say "baruch hamavdil ben kodesh lechol" or havdalah to be able to perform melacha? 2. Is one allowed to ask Hashem for things in such a shemoneh esrei (i.e. refuah for specific people, personal prayers in Shma Koleinu, etc.) since technically it is nightfall but it is still Shabbat in terms of performing melacha.

Shalom, It is permitted to daven ma'ariv a few minutes early on Saturday night, and one says ata chonantanu, which does allow you to do work when Shabbat goes out a few minutes later. And yes, you can make weekday requests in the Amidah, after saying the havdallah in the Amidah. Most opinions hold that it is a good thing to daven a little later on motzay Shabbat, in order to hold onto the Shabbat for as long as we can. But if there is good reason to daven in this earlier minyan, such as when this is the minyan in your shule, or when you need to rush to do a mitzvah on motzay Shabbat, it is certainly permitted to daven 10 minutes before the motzay Shabbat time, as long one remembers not to do melacha until the time for motzay Shabbat is reached. Blessings