1. The German class started. I arrived.
When I arrived the German class had already started.
2. I didn’t recognize her. We met a few times before.
Although we had met a few times before,I didn't recognize her .
3. She went bright red. She used the wrong word.
After she had use the wrong word,she went bright red.
4. Mike was late for the class. He left his books at home.
Mike was late for school because he had left his books at home .
5. Ana spent an hour in the advanced class. She realised she was in the wrong room.
Ana had spent an hour in the advanced class before she realized she was in the wrong room.