1．“进行时+when+一般时”表示“正在……，就在这时”

孩子们正要离开学校，这时他们看见了一辆卡车。

【误】the children left school when they saw a truck.

【正】the children were leaving school when they saw a truck.

2．while不能用于含有表示一时性动作的动词的句子

当我讲话的时候请安静。

【误】please be quiet while i speak.

【正】please be quiet while i am speaking.

3．表示一时性动作的动词，如：finish, return, start, leave, come, reach, go要用其否定式与till/until连用

我恐怕要在他来之前才能完成工作。

【误】i'm afraid i can finish the work when he comes.

【正】i'm afraid i can't finish the work until/till he comes.

表持续性动作的动词，如：stay, wait要用其肯定形式与till/until连用。

我呆在那里直到他回来。

【误】i stayed there when he arrived.

【正】i stayed there until/till he arrived.

4．because所引导的从句可放在句首；for连接的分句不能放在句首

他不害怕，因为他很勇敢。

【误】for he was very brave, he felt no fear.

【正】he felt no fear, for he was very brave.

because表示直接的原因；for表示推断的原因。

一定是早晨了，鸟在叫呢。

【误】it must be morning now, because the birds are singing.

【正】it must be morning now, for the birds are singing.

分析：早晨到来的直接原因是太阳升起，而决非是鸟唱歌。鸟唱歌只表示推断的原因。