`16.B`

`->` By the time + S + Ved/V2, S + had + P2

`17.D`

`->` S + was/were + V-ing + when + S + Ved/V2

`18.A`

`->` S + have/has not + P2 + for + time

`=` It's + time + since + S + last + Ved/V2

`19.B`

`->` S + was/were + V-ing + while + S + was/were + V-ing

`20.A`

`->` usually `->` thì HTĐ
`(+)` S + V-(s/es)

`21.A`

`->` hành động đang xảy ra thì một hành động khác chen ngang `->` thì QKTD

As + S + was/were + V-ing, S + Ved/V2

`22.D`

`->` `(?)` Have/has + S + P2 + since + S + Ved/V2?

`23.C`

`->` S + Ved/V2 + when + S + was/were + adj/Noun

`24.B`

`->` như câu `21`

`25.B`

`->` at this time yesterday `->` thì QKTD

`(+)` S + was/were + V-ing

`26.C`

`->` After + S + V-(s/es), S + will + V-bare