英文文法: 英文逗号与状语从句
Source: Zhihu
(prepared by: alexcwlin; edited by: Adam Lam)

Question:
状语从句为啥加逗号咩？逗号问题总是搞不懂?

alexcwlin comments:

A.    逗号的作用
逗号(comma)是把不同性质的元素分隔开.
例如: “我的邻居陈娇娇是可人儿”不需要逗号，因为我邻居众多，要阐明 “陈娇娇”才可以分辨出那一个邻居.
也即是: “我向东的邻居, 陈娇娇, 是可人儿”, 假定陈娇娇是独居，这句子她的名字只是附带资料不用拿来确认是那一个邻居, 所以需要逗号隔开.
当然有备受争议的灰色地带暂且不提。

B.    逗号与状语从句中句子位置
(粗体字是状语从句)

1.     开句(+逗号)

a.      Whenever you need help, I am always there for you.

b.     When I hear the song “San Francisco”, I always think of the carefree hippies.

2.     中间(+逗号x2)

a.      My friend John, when he runs out of money, always hits me for a loan.

b.     Penguins, although they have wings, are unable to fly.

3.     结尾(+逗号or -逗号)

a.      Yesterday he came home after I had left.
(备注: 从句 “after I had left” 在确定时间是必需所以不用逗号; 英文术语是”restrictive” or “essential”)

b.     Yesterday he came home at 5:00 p.m., after I had left.
(备注: 从句 “after I had left” 在确定时间是不重要, 所以要用逗号来分隔开主句; 英文术语是 “non-restrictive” or “non-essential”)

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