It was last night that he came back. 

上面这句话，that 改为 when 是可以的。即：

It was last night when he came back. 

It was last night when he came back. 该句有两种理解：

（1）仍然是强调句。如果被强调的成分是由“时间副词” 充当的时间状语，那么，可以用when 代替 that。如果被强调的成分是由“介词短语”充当的时间状语，那么，只能用that，不能用 when。本句 last night 是时间副词，可以用 when 引导强调句。

同理，如果被强调的成分是由“地点副词” 充当的地点状语，那么，可以用 where 代替 that。如果被强调的成分是由“介词短语”充当的地点状语，那么，只能用 that，不能用 where。

例如：　　

It was yesterday that (when) he bought an English book in town.

It was last week that (when) we worked in the countryside.　　

It is here that (where) he must wait for her.　　

It was in this room that we had a heated discussion.

It is at ten o’clock that the meeting will take place.

（2）跟第一句 It was 8 o'clock when he came back. 结构一样，是个主从句。 when 是时间从句，It was last night. 是主句。

意思是：“他回来的时间是昨天晚上。” 或 “他是昨晚回来的。”