Ответ:

He can't walk very fast because he has hurt his leg.

2 I can't get in because I have lost my key.

3 I know this story very well because I have seen the film.

4 I can't post the letter because I haven't put a stamp on it.

5 He can't stand up because he has eaten too much.

6 They can't go on holiday because they haven't saved money.

7 I know him because I have met him before.

8 We don't know how he is because we haven’t heard from him.

9 He won't take a cigarette because he has given up smoking.

10 She can't get in because he has locked the door.