`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.
