1. had left/arrived

2. wondered/had left

3. didn't leave/had finished

4. had entered/began

5. asked/hadn't done

6. had already posted/realized/had sent

@Holliwood#:

1. Almost everybody left by the time we arrived.

2. He wondering whether he leaving his wallet at home.

3. The secretary not leaving until she finishes her work.  

4. The couple scarcely entered the house when they begin. 

5. The teacher asking the boy why he not doing his homework. 

6. She already posting the letter when she realizes she sends it to the wrong address. 

Bn tham khảo nhé!!!!!!!!!!!!!!!!!!!!!!!

Xin ctlhn ạ!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!