Ответ:

1) began

2) have just come

3) visited

4) has already sold

5) learned

6) have not met

7) sent

8) understood

9) is

10) opened

11) bought

12) have you seen

13) wrote

14) keeps

15) switched

16) flown

17) have not finished

18) won

19) have not tasted

20) feels