Choose the correct alternatives in these sentences. Both alternatives may be possible.

1. I don't know how to / can't type, but it would be useful if I can / could.
2. If I don't get a job soon, I can't / won't be able to pay the rent.
3. I could swim / was able to swim by the time I was three.
4. My sister's succeeded in getting / managed to get a job with an international company.
5. Our final assignment was hard, but I managed to / could hand it in last week.
6. I'm sorry I couldn't / wasn't able to come to your party.
7. I could become / could have become a professional football player but I went to college instead.
8. I couldn't have helped / can't have helped you even if I'd wanted to.
9. It is able to / can rain in August, though it doesn't happen often.
10. My old boss could / managed to be very bad-tempered. ​