`1` The teacher told to the students that online learning had become more popular .

`-` Gián tiếp tường thuật .

`@` S + said/told + O + that + S + V(lùi thì) + O .

`-` has `->` had .

`2` John asked me whether recycling was the most efficient environmental protection method .

`-` Gián tiếp ''Yes/No'' .

`@` S + asked + O + if/whether + S + V(lùi thì) + O .

`-` is `->` was .

`3` Unless humans protect natural habitats , wild animals won't have place to live .

`@` Câu điều kiện diễn tả hành động có khả năng diễn ra trong tương lai .

`=>` Điều kiện loại `1` : If + S + V(s,es) + O , S + will/modal verbs + V-inf + O .

`-` Biến thể : Unless + S + V(s,es) + O , S + will/modal verbs + V-inf + O .

`4` If farmers didn't use too much pesticide , a lot of people in the world would have access to fresh water .

`@` Câu điều kiện diễn tả hành động không có thật ở hiện tại .

`=>` Điều kiện loại `2` : If + S + V(quá khứ)/were + O , S + would/modal verbs + V-inf + O .

`1.` The teacher told the students that online learning had become more popular.

`->` Câu tường thuật `-` kể:
S + told sb + that + S + V (lùi thì)

`-` Lùi thì: HTHT `->` QKHT

`2.` John asked me if recycling was the most eficient environmental protection method.

`->` Câu tường thuật `-` yes/no question:
S + asked sb + if/whether + S + V (lùi thì)

`-` Lùi thì: HTĐ `->` QKĐ

`3.` Unless humans protect natural habitats, wild animals won't have place to live.

`->` Điều kiện loại `1:`

If/Unless + S + V-(s/es), S + will/can/shall + V-bare

`-` Unless `=` If not: trừ khi...nếu không...

`4.` If farmers didn't use too much pesticide, a lot of people in the world would have access to fresh water.

`->` Điều kiện loại `2:`

`(-)` If + S + didn't + V-bare: S + would/could + V-bare