Exercise 5:                        

 1.since                          2. since

3. for                               4. since    

5.for                               6.since

7.for                                 8.for

9. since                            10. for

Exercise 6:        

1. have done               2.have written 

3.have never met        4. have left

5.haven't seen              6.has bought

7.has never seen           8.have been

9.have traveled             10.has told

EX5.

1. for       2.             3. for              4. since             5. for

6. since         7.for          8.for        9. since        10. for

EX6:        

1. have done               2.have written 

3.have never met        4. have left

5.haven't seen              6.has bought

7.has never seen           8.have been

9.have traveled             10.has told

EX7:

1. She has learned swim since last week

5. For me, football is the most interesting sports

4. This is the first time I have were to Da Lat