51a.
1 have to
2 had to
3 need
4 have to
5 need
6 must
51b
1 has to buy
2 have to find
3 must to go
4 Do we need to leave
5 must to clean
6 Did you have to pay
45a
1 Bob wasn't able to read until he was six.
2 We are able to see the sea from our window.
3 Not many people are able to drive a bus.
4 We were able to speak to them in English.
5 They weren't able to dance or sing.
6 I didn't able to understand the film.
52a
1 mustn't swim
2 aren't allowed to speak
3 mustn't touch
4 couldn't enter
5 can't use
6 aren't allowed to smoke
53a
1 a
2 f
3 e
4 d
5 b
6 c
51a.
1 have to
2 had to
3 need
4 have to
5 need
6 must
51b
1 has to buy
2 have to find
3 must to go
4 Do we need to leave
5 must to clean
6 Did you have to pay
45a
1 Bob wasn't able to read until he was six.
2 We are able to see the sea from our window.
3 Not many people are able to drive a bus.
4 We were able to speak to them in English.
5 They weren't able to dance or sing.
6 I didn't able to understand the film.
52a
1 mustn't swim
2 aren't allowed to speak
3 mustn't touch
4 couldn't enter
5 can't use
6 aren't allowed to smoke
53a
1 a
2 f
3 e
4 d
5 b
6 c