`12` was - had been

`=>` S + V2/ed + until + S + had past participle

`=>` ago `->` QKĐ

`=>` until 2 months ago `->` QKHT

`13` broken

`=>`Thì QKĐ: Với động từ thường: S + V2/ed

`=>` DHNB: the day before her exam

`14` catches

`=>` every winter: thì HTĐ

`15` did he travel

`=>` yesterday: thì QKĐ

`16` was waiting - was waiting

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

`=>`  nói về những hành động xảy ra liên tiếp, có thể diễn ra trong 1 khoảng thời gian

`17`  was leaving

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

`18` didn't see

`=>` last monday: thì QKĐ

`19`sailed

`=>` ago `->` QKĐ

`=>` S+Ved-2

`20` will leave / knows 

`=>` I think: tôi nghĩ `->` thì TLĐ

`=>` S+will+V

`=>` Vế sau là HTĐ: S+Vs/es

`21` was rainning - decided

`=>` last week: quá khứ đơn

`=>`Vế đầu là QKTD `->` `(+)` S + was/were + V-ing

`=>` Vế sau là QKĐ `->` `(+)` S+Ved/2