`1.` has studied
`->` Có was nên ta dùng HTHT
`2.` have studied
`->` Dấu hiệu: for one month
`=>` HTHT
`3.` have worked
`->` Dấu hiệu: since `1970`
`=>` HTHT
`4.` Have you learned
`5.` Have you met
said
`6.` have been waiting
hasn't come
`7.` has written
`8.` have you learnt
`9.` have already finished
`10.` have been
`11.` has never been
`12.` has studied
`13.` felt
`->` QKĐ
`->` Dấu hiệu: last
`14.` have studied
`->` Dấu hiệu: since `1980`
`=>` HTHT
`15.` bought
have driven
`=>` Ta có công thức HTHT như sau:
`(+)` S + have/has + V_3/V_ed
`(-)` S + have/has + not + V_3/V_ed
`(?)` Have/has + S + V_3?
`=>` Ta có công thức QKĐ như sau:
`(+)` S + V_ed
`(-)` S + didn't + ...
`(?)` Did + S + ....?