1.Mary sat down(beside) me (a)moment, after Tom was （gone）
beside 不用改
a改为for a
gone 不用该
Mary sat down (beside) me (for a) moment, after Tom was （gone）
2.（Would) you come (to) help me? I'm very busy (in) the moment.
would 不用改
to 改为 and
in 改为 at
（Would) you come (and) help me? I'm very busy (at) the moment.
希望帮到了你.