Question

原因状语从句中，为什么because与主句不用逗号隔开而as要用?

Answer 1

because引导原因状语从句语气最强，表示直接原因,常用来回答由why提出的问题，所引导从句可放在主句前，也可放在主句后。
例如：
"Why did you do it?""Because Carlos told me to".
We can't go to Julia's party because we'regoing away that weekend.
Because he didn't catch the last bus, hehad to walk home.
Why are you late for school ? Because I missed the first bus.
as “由于,鉴于” 主从并重，从句说明原因，主语说明结果。说明比较明显的原因。多用于日常语中，常译为“由于”，从句一般放于主句前。
例如：
You can go first as you're the oldest.
As you don't feel well, you had better stayat home.
As he wasn't ready in time, we went withouthim.
I must stop writing now, as I have rather alot of work to do.
As it was getting late, I decided to bookinto a hotel.
As it is raining, you'd better take a taxi.
拓展：since和for也能表示原因
since “因为,既然” 侧重主句，主句表示显然的或已知的理由。强调说话者和听者双方都知道的原因，常译为“既然”，一般用在这样的句子里。
例如：
Since we've got a few minutes to wait forthe train, let's have a cup of coffee.
Since you are not feeling well, you maystay at home.
Since everybody is here, let's begin themeeting.
Since I am a boy, let me carry the case.
Since/As you are busy, you had better notgo with us.
Since we have no money, we can't buy thatvase.
for “因为,由于”表明附加或推断的理由，引导的分句前常有逗号。for分句不放句首.是用来附带解释或说明前一分句的原因和理由。
例如：
Day breaks, for the cock crows.
It must have rained last night, for theground is wet.
I won't go in, for my father is there.
He must be ill, for he is absent.
I went to see him, for I had something totell him.

Answer 2

1. because 作为连词，连接主句与从句，连接从句可以与主句相连，也可与主句分开，而 as 在引导状语从句时，从句既可与主句相连，也可与主句分开。

   例：

   (1). She was praised by her teacher as she completed the task in advance.

   　   As she completed the task in advance, she was praised by her teacher.

   (2). She was praised by her teacher beacuse she completed the task in advance.

   　　She was praised by her teacher , beacuse she completed the task in advance.

2. 主句与从句用逗号分开常见于定语从句中的非限制性定语从句。
   

　　例：

　　She completed the task in advance, which makes her praised by her teacher.