21. was built in the 18th century

bị động thì quá khứ đơn: was/were Vp2

22. was written by Clarke Moore in 1823

tương tự 21

23. will be invited to the party

bị động thì tương lai đơn: will be Vp2

24. she had to finish her work before 7pm

must sang gián tiếp => had to

25. my dad would come there in half an hour

sang gián tiếp lùi 1 thì

26. worked for this company for three years

hiện tại hoàn thành: have/has Vp2 for + khoảng thời gian

27. seen her for three years

tương tự 26

28. played since she was three

mệnh đề sau since chia quá khứ đơn

29. Hoa if Phong Nha Cave was in southern Viet Nam

asked sb if + mệnh đề để dạng khẳng định và lùi 1 thì

30. drew the picture for thirty minutes

tương tự 26