Here are the completed sentences using the correct forms of must, have to, need, and the verbs in brackets:

1. You **don't have to pay** me back now, but of course you can if you like.
2. Dogs **must be kept** on a leash at all times.
3. If Linda **isn't picked up** from school tomorrow, maybe we could spend the afternoon in town.
4. We **won't need to rely** on phone lines in the future as we'll all have mobile phones which connect to the Internet.
5. The invitations **have to be sent off** soon otherwise they won't get there in time.
6. Sorry I'm late. I **had to get** a few things from the supermarket on my way here.