1. We / discussing / many things / when / the headmaster / come in.

=>We were discussing many things when the headmaster came in

2. When / the teacher / come in, Lam / cleaning / blackboard.

=>When the teacher came in,Lam was cleaning the blackboard

3. When / the telephone / ring / I / read / novel. .

=>When the telephone rang,l was reading a novel

4. While / we / play / football, they / swim / the river. 

=> While we was playing football,they was swiming in the river

5. When my dad / mend / the roof / my mum / cook / lunch. 

=>When my dad mended in the roof, my mum cooked lunch

Chúc bạn học tốt!

`1` . We were discussing many things when the headmaster came in.

`2` . When the teacher came in, Lam was cleaning the blackboard.

`3` . When the telephone rang, I was reading a novel.

`4` . While we were playing football, they swam in the river.

`5` . When my dad was mending the roof, my mom cooked lunch.

*Giải thích :

Câu `1` + `2` + `3` + `5` đều cùng là một câu có 1 sự việc đang diễn ra (chia quá khứ tiếp diễn) thì một hành động khác chen vào (chia quá khứ đơn)

Câu `4` thì while + thì quá khứ tiếp diễn , thì quá khứ tiếp diễn

(Tức là cả 2 vế của while đều chia quá khứ tiếp diễn như câu mình viết bên trên)