Question

it's the first time that sb. have done到底是不是强调句

Answer 1

不是强调句。判断强调句与定语从句很简单，把it be与that同时去掉，看剩余部分能否成句。
比如LZ这题，去掉后，变为sb. have done sth. (for) the first time，不能独立成句（少一for）。
另外。何必纠结这些概念。
如有不懂可追问。
欢迎采纳O(∩_∩)O哈哈~

追问

本来我从没有纠结过这类问题，但是我朋友硬要说它是强调句
并且对the fiest time 用法也不是很清楚

追答

the first time 常引导一个从句， 如：
The first time time I saw you, I realized I semmed to have met you somewhere before.相当于as soon as 。
This is the first time I've been here.
for the first time,第一次做某事, 不要加从句.常用现在完成时.
I have been here for the first time.

Answer 2

不是强调句

Answer 3

是啊

追问

为什么呢。比如说，it's the first time of the year that i have seen such a sunset,
如果拆开来看：i have seen such a sunset the first time of the year 这个句子是完整的吗？
the first time 前面不用加介词吗？