题目内容：

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

优质解答

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.
希望我的答案能帮到你.