after必须用在过去完成时中吗?
不一定.但你给的例句也不对.以下的修改同时也回答了第一个问题：
After she finished writing the letter,she then went to the post office.
问题补充的例句：
He went to the US when he was 14 years of age.
After/As soon as he finished dinner,he went back to work.
He did not go to sleep until it was midnight.
By that time,he was only 11,too young to understand.