Then came the day for which he had to leave.

这句话的意思是：到了必须走的日子了。这句话的主句是then came the day，定于从句是for which he has to leave。它修饰先行词the day，the day表示时间。这句话是想用“介词 which”来引导。这个病句出错就在于这个介词上。

我们现在把the day放入定语从句中，您看这个空格处填写哪个介词。He had to leave the day. 表示”在某天“，我们用介词on，即：on the day。那么，病句中的定语从句就要用“on which”来引导。

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此外，on which可以用that来代替。因此这个病句可以有三种方式来改正：

Then came the day on which he had to leave.

Then came the day when he had to leave.

Then came the day that he had leave.

进而，我们还可以把以上三个句子中的引导词都省略去，可以这么说：

Then came the day he had to leave.