This is your first time

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

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.