1. While he was walking in the street, he saw the accident happen. （当 ... 时）
While I admit he is smart, I don't like him.(虽然，尽管）
2. When he was still a little boy, his father died from an accident.
When I saw the pictures, I couldn't help crying.
3. Since you are eighteen, you should learn to be responsible for your own deeds. (既然）
Since you can't answer my question, I will ask someone else.
（since引导时间状语从句时多在句中，不在句首）
4. We were surpeised that he was late, for he was always punctual.
He can't go there with us, for his mother is ill seriosly.
5. As I was getting on the bus, I realized that I had left my purse at home.（当 ...时）
As is mentioned above, he never came back to his hometown. (正如...那样，引导非限制性定语从句）
6. He failed the exam because he was too lazy and he never studied hard.
We couldn't go out because it was too cold.
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