1. The teacher told us that scientists had searched for extraterrestrial life on other planets for decades.

- Lùi thì: Thì hiện tại hoàn thành -> Thì quá khứ hoàn thành

               have searched              -> had searched

* Cấu trúc câu tường thuật chung:

   S + said (that) / said to sb that / told sb that + S + V(lùi thì) + ....

2. It's not easy / It's difficult to eat in weightless conditions.

* Cấu trúc: It + be + (not) + adj + to + V 

3. Aliens might invade the Earth and do the same thing

- Might :  thường dùng với nghĩa ít chắn chắn. khả năng xảy ra không cao.

* Cấu trúc: S + might + V_infinitive

1. The teacher told us that scientists had searched for extraterrestrial life on other planets for decades.

2. It's difficult to eat in weightless conditions.

3. Aliens might invade the Earth and do the same thing.