1. This is the third time I have asked you this question ( thì hiện tại hoàn thành)

2. Social network has been used for many purposes by teenagers these days.

( thể bị động của thì hiện tại hoàn thành: S+ has/have been+ V_ed/3 )

3. I suggested that saving-energy appliances should be used instead of ordinary ones to save energy. ( câu giả định )

4. If they were strong enough, they could lift the heavy door.

( câu điều kiện loại II: If+ S+ QKĐ, S+ would/could/ might+ V_inf )

1. This is the third time I have asked you this question 

This is the first/second... time S have/has VPII.

2. Social network has been used for many purposes by teenagers these days.

Bị động thì HTHT: S+ have/has been+ VPII

3. I suggested that saving-energy appliances should be used instead of ordinary ones to save energy. 

4. If they were stronger, they could lift the heavy door.