为什么when在引导表语从句和定语从句中是副词在时间状语从句中是连词?

when除了用来引导主语从句、宾语从句、时间状语从句、表语从句和定语从句，可以为副词、连词、代词，根据不同用法而定。
下面是when的不同词性用法，差别还是挺大的，看下例句，就能感受出来。
语法没有为什么，就是这么用的，应用熟练就好了。
adv副词，conj连词 pron 代词
when:
(1) adv. 什么时候，何时 at what time
[例句]
When can you come to see me?
你什么时候来看我？
(2) adv. 是…时候at which;on which
[例句]
Please tell me when we'll have the meeting.
请告诉我我们什么时候开会。
(3) adv. 在…的时候 at or during which
[例句]
How I regretted the days when I had played and should have studied!
我多么懊悔我本该好好学习却玩掉了的那么多的日子！
(4) adv. 那时 but at that moment
[例句]
It happened on a day in November when the weather was very wet and cold.
这事发生在十一月的一天，那时天气又潮湿又寒冷。
We were about to start,when it began to rain. 我们刚要出发时开始下雨了。
(5) conj. 当…时 at the time when
[例句]
It happened when I was living in the country. 事发生在我住在农村的时候。
It was raining when we arrived. 我们到达时，天正下着雨。
(6) conj. 一…就 immediately after
[例句]
When I had finished dinner I went to bed.
我吃完饭就睡觉了。
Let us set out when the sun has risen. 太阳一出来，我们就动身。
(7) pron. （关系代词）那时
which time
[例句]
I came here in 1949, since when I have been engaged in this work.
我1949年到这里，从那时起我就担任这项工作。
We came a week ago,since when the weather has been bad.
我们在一星期前来此，从那时起天气就一直不好。
(8) pron. （疑问代词）什么时候
what time
[例句]
Since when have you been here? 你从何时起在此地？
Until when can you stay? 你能呆到什么时候？