It's the first time ,for the first time ,the first time ,at first 区别?哪个后面加过去时态

1.It's the first time that I have been here. 或It was the first time that I had been there.注：It's the first time that+现在完成时， It was the first time that+过去完成时。
2.for the first time 只能作状语，一般用于一般过去时，例：I went to Beijing last year for the first time.
3.the first time是高中重点语法，即名词作连词，类似的还有the moment , the minute等，译为：一```就```，多与一般过去时连用。例：I fell in love with Kunming the first time I came here.
4.at first 只作时间状语，意为：开始，首先。多与一般过去时连用。例：At first , I have no interest in English.

只有在句子中 才能说清楚
it is the first time 后面经常跟that 引导的定语从句
for the first time 经常和现在完成时 连用
the first time 就是一个名词词组
at first 是介词短语 起副词作用 作状语 经常跟 一般过去时连用