16. James (weed) while Alex (cut) the grass in the garden this time yesterday. 17. Susan (wait) for you at 9 o'clock tomorrow. 18. By next summer, she (finish) her 3rd novel. 19. This is the first time we (see) the Statue of Liberty. 20. By the time Anne graduated from high school, she (attend) seven different schools because her parents moved frequently.

1. James (weed) while Alex (cut) the grass in the garden this time
2. Susan (will be wait) for you at 9 o'clock
3. By next summer, she (is finishing) her 3rd
4. This is the first time we (have seen) the Statue of
5. By the time Anne graduated from high school, she (had attended) seven different schools because her parents moved

16. has weeding - was cutting

Cấu trúc: S + was/were + V_ing while + S + was/were + V_ing.

17. will wait

Tomorrow, thì tương lai đơn, cấu trúc: S + will + Vo.

18. will finish

By next summer, mùa hè năm sau, mang tính tương lai, nên dùng thì tương lai đơn.

20. attended

Vì vế trước là thì quá khứ đơn, nên vế sau cũng vậy