This is your first time ---you tell us the story 在这个句子中是用关系代词that 还是用when 为什么用that 在句中做什么成分

用when
因为定语从句结构完整，用关系副词。

用when
不用that.
用that的话,本句应看成为强调句,通常应为
it is ... that 的格式
这句也可改写为
it is your first time that you tell us the story.

这是个包含了关系副词引导的定语从句的复合句.
因为从句的主、谓、宾都有,所以不能够用"That".只能够用“When”或者“in which”
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四．关系副词引导的定语从句
1. when指时间,在定语从句中做时间状语
(1) I still remember the day when I first came to the school.
(2) The time when we got together finally came.
2. where指地点,在定语从句中做地点状语
(1) Shanghai is the city where I was born.
(2) The house where I lived ten years ago has been pulled down.
3. why指原因,在定语从句中做原因状语
(1) Please tell me the reason why you missed the plane.
(2) I don’t know the reason why he looks unhappy today.
注意：关系副词引导的从句可以由“介词+关系代词”引导的从句替换
(1) The reason why/ for which he refused the invitation is not clear,
(2) From the year when/in which he was going to school he began to know what he wanted when he grew up.
(3) Great changes have taken place in the city in which./where I was born.