一，简单句（simple sentence）
简单句只由一个独立子句构成，它至少包含一个主语和一个动词。它还可以包含宾语和修饰词。
例句：
Birds fly.
Studies serve for delight, for ornament, and for ability.
Perhaps the decline of this country has already started.
I was wearing my powder-blue suit, with dark blue shirt, tie and display handkerchief, black brogues, black wool socks with dark blue clocks on them.
从上面可以看出来，简单句有的非常简单，有的看上去也挺复杂。这是因为简单句也分为两种类型：简单的简单句，和复合简单句。
1）简单的简单句
其实就是只有一个主语和一个动词的句子。
例句：
He sold it for a high price on Amazon.
ou have to dream to make your dream come true.
2）复合简单句
这种句子包含一个主语和多个动词，或者多个主语和一个动词。
例句：
The dog barked and ran.
Julia and Mary hired a taxi to the airport.
二，复合句（compound sentence）
复合句由至少两个独立子句构成，中间由连接词构成，也可以由分号、冒号、破折号连接。
连接词包括：
例句：
Only two things are infinite, the universe and human stupidity, and I'm not sure about the former.
There used to be a real me, but I had it surgically removed.
She did not cheat on the test, for it was the wrong thing to do.
I am counting my calories, yet I really want dessert.
（分号）Joe made the sugar cookies; Susan decorated them.
（分号）She works two jobs to make ends meet; at least, that was her reason for not having time to join us.
（冒号）I know one thing: I love that girl.
（破折号）I know you're here — I can smell your perfume.
注意1：
如果复合句中有连接词，那么连接词前面必须加逗号，比如：
I really need to go to work, but I am too sick to drive.
如果是：
I really need to go to work but I am too sick to drive. 那么是错误的。
有的句子中，连接词前面没有逗号，说明后面跟着的不是独立子句，而是从属子句，整个句子就是复杂句，而不是复合句。
比如：
A geek is a guy who has everything going for him but is just too young. He's got the software but doesn't have the hardware yet.
这句话不会长成这样：
A geek is a guy who has everything going for him, but is just too young. He's got the software, but doesn't have the hardware yet.
注意2：
关于两个独立子句能不能单纯靠逗号组成复合句。这个，根据The sense of style和其他语法书籍上的说法，是不能的，这是一种常见的英文写作错误。但是在实际的文学写作中，这很常见，比如：
Flo scorned slacks, she scorned the outfits of people trying to be in style, she scorned lipstick and permanents.
这句话是由诺贝尔文学奖得主Alice Munro在短篇小说Royal Beating中出现的。
还有：
Help is what they offer but gratitude is what they want, they roll around in it like cats in the catnip.
这句话出现在Margaret Atwood的长篇小说《双面格蕾丝》中。
因此，我觉得，如果是文学写作，那么用逗号连接两个独立子句是没问题的，但如果是学术写作，还是避免这样为好。
三，复杂句（complex sentence）
复杂句由一个独立子句以及至少一个从属子句构成，它们之间由连词连接。
在复杂句中，从属子句往往是对独立子句起到一个解释说明的作用，告诉读者更多的信息。它或是表现出一种因果关系，或是表现在时间或者空间上的关系。
1）表现因果关系的例句：
Wise men speak because they have something to say. Fools speak because they have to say something.
Batman required strict compliance with seat belt rules, hence Robin was not allowed to ride in the Batmobile.
Since Robin refused to wear his seat belt, Batman has banned him from the Batmobile.
I am extremely happy since I retired.
2）表现时间/空间关系的例句：
Once Batman learned that Robin had not been wearing his seatbelt, he took away his keys to the Batmobile.
Robin looked regretfully at the Batmobile whenever he passed it in the Batcave.
After Batman was done working for the night, Robin took a secret ride in the Batmobile.
Many years later,as he faced the firing squad, Colonel Aurelian Buendia was to remember that distant afternoon when his father took him to discover ice.
注意：
一般情况下，在复杂句中，如果从属子句的位置靠前，并且作为句子中的副词使用时（副词就是用来表达地点、时间、程度等信息），那么它和独立子句之间需要加一个逗号；如果从属子句的位置靠后，那么它和独立子句之间不需要加一个逗号。
从属子句在前的例子：
When I lost all my friends, I became a people-watcher.
从属子句在后的例子：
Don't use a comma when your adverbial is at the back. （没有逗号）
但是，如果从属子句中表达的意思和独立子句有着强烈的反差，或者作者想重点强调从属子句中的内容，那么当它的位置靠后时，它前面也可以加逗号。
比如：
Annie was still crying, although she had been happy about the news.
四，复合复杂句（compound-complex sentence）
复合复杂句由至少两个独立子句和至少一个从属子句构成，子句和子句之间用连接词相连。
例句：
（黑体部份为两个独立子句，斜体部份为从属子句。）
Kate doesn’t like cartoons because they are loud, so she doesn’t watch them.
The cat jumped onto the couch and sat down on top of the remote control just when I was reaching for it.
关于如何在复合复杂句中区分独立子句和从属子句：
主要看内容的重要程度。在句子中，有的部份是对整体内容进行零碎的信息补充，它们就是从属子句，在重要性上没有独立子句们那么重要。
并且，独立子句们的主语往往是同一个，比如上面的两个例句中，Kate和Cat是独立子句们的主语；而“they”（这里指向“cartoons”）和I则是从属子句的主语。（当然这只是大部分情况，主要还是看重要性）
从属子句可以出现在复合复杂句的开头、中间，或者结尾。
例句：
（黑体部份为两个独立子句，斜体部份为从属子句。）
（开头）Even if the child is hungry, he will never eat oatmeal, but he will always eat ice cream.
（开头）Though my mother says it doesn't matter, I am tall, and she is short.
（中间）It is important to vote when the time comes, or you won't get a say in new laws.
（中间）Usually I take a walk every day while the sun sets, but it was raining today.
（结尾）She likes to sleep in but she can get up early if she has work.
（结尾）The dog needed a new leash, and he couldn't go for a walk until he had one.
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