Ответ:
1. I have just finished my homework.
2. Mary has already written five letters.
3. My friend was in Canada two years ago.
4. I have not been to Canada so far, but I have already travel(l)ed to London a couple of times.
5. I can't take any pictures because I have not bought a new film yet.
6. Did they spend their holiday in New Zealand last summer?
7. Have you ever seen a whale?
8. I have already written two essays this term.
9. I spent my summer holiday in Australia last year.
10. The floor looks dirty. I'm sure you have not cleaned up.
11. My bicycle isn't here. I think someone has just taken it.
12. They have known each other since they were children.
13. I have been a member of this non-profit organization since 1996.
14. She has made many new friends since she joined this English club.
15. How many games has your team won so far this season?
16. I have not seen Peter since I arrived.