1. The lesson must be studied (It's + important to V `->` S + must be ++ Vp.p)

2. Bị động kép: S + think/say/believe/...(V) + that + Clause (QKĐ, HTHT)

`->` It + tobe + thought/said/believed/...(Vp.p) + that + Clause

`->` S (S in Clause) + tobe + thought/said/believed/...(Vp.p) + to have Vp.p

- It was thought that you were selfish

- You were thought to have been selfish

`1.` The lesson must be studied.

`-` It's important + to V-inf: Điều quan trọng là phải....

`-` Chuyển đổi sang câu must: `( + )` S + must + V-inf + ... `->` Chuyển sang câu bị động với must:

`( + )` S + must + be + VpII + (by + O) + ....

`2.` `-` Bị động kép (bị động chứa that)

`-` Form: Active: S + believed/thought/said/told/... 

`->` Passive: `-` It is/was + believed/thought/said/told/... + that + S + V + ... (V chia theo thì) 

`ANS:` It was thought that you were selfish.

`-` S + be (am/is/are/was/were) + believed/thought/said/told + to V-inf (cùng thì) | to have + VpII (khác thì)

`ANS:` You were thought to have been selfish.