=>
21. will have already bought
-> when HTĐ, TLHT
22. doesn't spend
-> each year (HTĐ)
23. didn't go
-> when she was ill (QKĐ, S V2/ed)
24. have studied
-> for 4 years (HTHT, S have/has V3/ed)
25. circles
-> every 365 days (HTĐ, S Vs/es)
26. read
-> yesterday (QKĐ)
27. have studied
-> recently (HTHT)
28. are working
-> now (HTTD, S is/am/are Ving)
29. didn't listen
-> yesterday (QKĐ)
30. is playing
-> at the moment (HTTD)