It was in 1990 ------he began to work.It was 1990 ------he began to work.用when/that填空

It was in 1990 that he began to work. It was 1990 when he began to work.
前面一句是强调句；后面一句的when是引导时间状语的。他开始工作的时候是1990年。

that
when

that ，强调

that when

It was in 1990 that he began to work.It was 1990 when he began to work.
前面一句是强调句型,强调句强调的是完整的时间,在1990年,要有介词In.而后面一句由于没有介词In,只有1990,所以不能看成是时间强调句型,只能是从句,而这里出现了1990,所以是时间状语从句,需要一个引导词,故而填when.
注：强调句和从句的区别是：强调句去掉"It was/is.that"这几个单词,仍然是一个完整的句子,而从句去掉以后句子不完整,通过这种方法,你可以看到第一句去掉It was、、、that 后就是in 1990 he began to work,这仍然是一个完整的句子,而后面那句去掉以后剩下的就是1990,he began to work.这不是一个完整的句子,在哪一年要用介词in,而它恰好就缺.
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