Question

改错:1、They didn't see each other

改错:1、They didn't see each other since September.

Answer 1

改为：
they haven't seen each other since september.
自从去年九月后他们一直没见面。
这里应该使用现在完成时，强调过去的事对现在造成的影响。

Answer 2

They haven't see each other since September.

Answer 3

didn't 改成have not
since做状语的话，需要用完成时