我以为你已经见过莎拉了。

I was ___under ___the ___impression you had already met Sarah.

她以为会议是在下午三点开始。

She was ___under ___the ___impression that the meeting started at 3 p.m.

我们以为博物馆每逢周一闭馆。

We were ___under ___the ___impression that the museum was closed on Mondays.