when conj.
when并列连词，= and then，表示“就在那时，突然”，常见以下句型中：
（1） be doing…when…
正在做……突然……
I was wandering through the streets when l caught sight of a tailor's shop.
（2） had done…when…
刚做了……突然……
I had just sat down when the light went out.
（3） be about to do…when…
刚要做……突然……
I was just about to go swimming when our guide saw me and shouted at me.
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