Question

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

Answer 1

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。

Answer 2

that
when

Answer 3

that when

Answer 4

that ，强调

Answer 5

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。