看情况,由于when可以引导时间状语从句(表示 当...的时候)和宾语从句(表示什么时候),所以:
1. 引导时间状语从句时:当主句是过去时态,从句一致,用过去时态,当主句是将来时态或者祈使句以及谓语由情态动词+动词原形构成,则从句用一般现在时.
They were talking when the teacher came into the classroom.
The boy will be a writer when he grows up.这个男孩长大后将成为一名作家。
When the lights are red, the traffic must stop.红灯亮时,车辆必须停下。
2. 引导宾语从句,没有限制,根据句子意思的需要用相应的时态:
I don't know when he will come back tomorrow.
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