after放句首的时态

after做介词时，后面只能加名词、代词、动名词短语，形成介宾短语，做句子的状语；after后面直接加一个动词时，它作为一个介词，动词用ing形式。after的前后时态遵循“主将从现”原则。

由 after,before,和 when 引导的状语从句在复合句中的\'位置比较灵活,放在句首或句末通常不会影响原句的意思

After I called you,I put it back = I put it back after I called you.我给你打电话后，把它放回去了。

Before I came,did anything happen?= Did anything happen before I came?在我来之前，有什么事发生吗？

When I saw my friend,he was eating.= My friend was eating when I saw him.当我看到我的朋友时，他正在吃饭。