It was 3 years ago ______ he came back.

Question

我知道答案是after，但不知道原因，请知道的告诉我，非常感谢
原题是:用before after since填空
It was 3 years ______ he came back.
It was 3 years ago ______ he came back.
It is 3 years ______ he came back.

Answer 1

也可以填when的吧？
另一种说法，It had been three years before he came back.
It has been three years since he came back.

Answer 2

最佳答案： It was 3 years before he came back.等他回来，已经三年过去了。
相当于：3 years has passed when he came back.
第二句话最好是用when, It was 3 years ago when he came back.
他回来， 那是三年以前(的事)了
It is 3 years since he came back.自从他回来，（至今）已经是三年的时间了。

Answer 3

他回来是三年前，所以要用过去式 不能用since用since的话前面要过去完成时态
所以
after

Answer 4

我觉得应该是when吧……填after怎么觉得有点怪呢……不是时间点吗。答案没错吧，