1. They arrived very late because they had a flat tire. *
=> If
2. I advise you not to say anything like that again. *
=> You’d
3. You remember to post the letter, didn’t you? *
=> You didn’t
4. “I’m sorry I gave you the wrong number”, said Paul to Susan. *
=> Paul apologized
5. Please don’t smoke in the kitchen. *
=> I’d rather
1. They arrived very late because they had a flat tire. *
=> If they hadn't had a flat tyre, they wouldn't have arrived late.
2. I advise you not to say anything like that again. *
=> You’d better not say anything like that again
3. You remember to post the letter, didn’t you? *
=> You didn’t forget to post the letter, did you ?
4. “I’m sorry I gave you the wrong number”, said Paul to Susan. *
=> Paul apologized to Susan for giving the wrong number.
5. Please don’t smoke in the kitchen. *
=> I’d rather you didn't smoke in the kitchen.