Why は instead of で？


My friend correct one of my essay sentences from:

> 大学を卒業した後で、日本へ帰ってJLPT1を取るために勉強します。

to

> 大学を卒業した後は、日本へ帰ってJLPT1を取るために勉強します。

What is the difference between them and why is は better? Is it used for
emphasis and if so why?




In the first sentence, 大学を卒業した後（で） is not marked with は, which means that part
is a new information or the focus of the remark, in other words, it's like an
answer to question "when do you go back to Japan and study for JPT1?". If you
explicit it, you can express it as "it's after I graduate from my college that
I go back to Japan and study for JPT1".

In the second sentence, 大学を卒業した後 is marked with は*, which means it's a shared
information or a precondition for the new information (日本へ帰って…勉強する), and you
can think of it as a kind of answer to a question "what will you do after
graduating from your college?".

*. More accurately, the point is it being the topic of the sentence. Being marked with は is only one of several ways to express that.




In the first sentence, 大学を卒業した後（で） is not marked with は, which means that part
is a new information or the focus of the remark, in other words, it's like an
answer to question "when do you go back to Japan and study for JPT1?". If you
explicit it, you can express it as "it's after I graduate from my college that
I go back to Japan and study for JPT1".

In the second sentence, 大学を卒業した後 is marked with は*, which means it's a shared
information or a precondition for the new information (日本へ帰って…勉強する), and you
can think of it as a kind of answer to a question "what will you do after
graduating from your college?".

*. More accurately, the point is it being the topic of the sentence. Being marked with は is only one of several ways to express that.

