Find the mistakes in the following sentences and correct them.

1 I couldn't have spoken English when I was at school, but now I'm very fluent.
2 It must to have been you that Frankie was talking about.
3 He must to have do it. There was nobody else there.
4 It can't be Louis who told you. He didn't know about it.
5 You should told him the whole story. He needed to know.