Вчера, Жанна покупала 2 сумки. - yesterday, Zhanna was shopping for 2 bags.
Perfect.

Вчера, Жанна купила 2 сумки. - yesterday, Zhanna bought two bags.
OK

сегодня Андрей пробежал марафон. - Today Andrei finished running a marathon.
Actually he both started and finished it today. I am not sure how to say it in English in a best way.

посмотрим фильм с тобой. - We'll watch the entire movie with you.
Mostly prefix по- means "to do smth. a bit".

Будем смотрим смотреть фильм с тобой поздно. - We'll watch the movie for a while later late (in the night).
for a while later = позже

Many say that Russian verb aspect is very difficult and contradictory. You better rely on intensive practice than on rules.