the first time,for the first time,by the first time怎么区别呢?

很简单
1 the first time (是连接词,用于主将从现和主过从过的句子中),
I was a boy of 16 THE FIRST TIME(=when) I came to this place (主过从过)
==I was a boy of 16 WHEN I came to this place FOR THE FIRST TIME.
You will like him THE FIRST TIME(==when) you see him next week(主将从现)
==You will like him WHEN you see him FOR THE FIRST TIME next week.
2 for the first time （第一次,往往用于简单句）
In 2009,he came to Shanghai for the first time
3 by the first time (翻译：在第一次以前)
Those people had heard so much about him BY THE FIRST TIME they saw him himself last year.(那些人在去年见到他本人以前就已经听说过他的许多事情)

the first time 连词，后面引导一个时间状语从句
for the first time 介宾词组作状语
by the first time 不是一个固定词组，可能是by+the first time+从句