简单句是只包含一个主谓结构的句子，通常由主语、谓语和宾语（可选）组成。
示例： Students study English.
简单句是构成复杂句的基础，通过组合和扩展简单句，可以形成更复杂的句子结构。
复杂句是包含一个主句和一个或多个从句的句子。从句通过从属连词与主句连接。
示例： Students study English because it is important for their future.
在这个例子中，"Students study English"是主句，"because it is important for their future"是从句，由从属连词"because"连接。
从属连词是连接主句和从句的重要词汇，它表明了从句与主句之间的逻辑关系。
常见从属连词：
从属连词的作用：
时间状语从句用于表示动作发生的时间，常用的从属连词有when, while, as, after, before, since, until等。
示例1： When I arrived at the station, the train had already left.
示例2： While she was studying, her brother was watching TV.
注意：when可以表示时间点或时间段，while通常表示时间段，as则强调两个动作同时进行。
原因状语从句用于表示动作发生的原因，常用的从属连词有because, since, as, for等。
示例1： She didn't attend the meeting because she was sick.
示例2： Since you're here, you can help me with this task.
注意：because表示直接原因，语气最强；since和as表示已知的原因，语气较弱。
条件状语从句用于表示动作发生的条件，常用的从属连词有if, unless, provided that, in case等。
示例1： If it rains tomorrow, we will cancel the picnic.
示例2： You won't pass the exam unless you study hard.
注意：if引导的是真实条件句，unless相当于"if not"，表示否定条件。
让步状语从句用于表示与主句内容相反的情况，常用的从属连词有although, though, even though, despite the fact that等。
示例1： Although it was cold, he went out without a coat.
示例2： Even though she is young, she is very mature.
注意：although和though可以互换使用，但though可以用于句末，而although不行。
目的状语从句用于表示动作的目的，常用的从属连词有so that, in order that, in case等。
示例1： She studies hard so that she can get a good job.
示例2： He left early in order that he could catch the first bus.
注意：so that和in order that可以互换使用，但in order that语气更正式。
结果状语从句用于表示动作的结果，常用的从属连词有so...that, such...that等。
示例1： She was so tired that she couldn't keep her eyes open.
示例2： It was such a beautiful day that we decided to go for a picnic.
注意：so后面接形容词或副词，such后面接名词短语。
关系代词用于引导定语从句，指代先行词并在从句中充当主语、宾语或定语。
关系代词的用法：
示例：
关系副词用于引导定语从句，指代先行词并在从句中充当状语。
关系副词的用法：
示例：
限制性定语从句是对先行词进行必要的限定，去掉后会影响句子的基本意思。
示例： The book that I borrowed from the library is very interesting.
在这个例子中，定语从句"that I borrowed from the library"限定了是哪一本书，如果去掉，句子的基本意思就不明确了。
非限制性定语从句是对先行词进行补充说明，去掉后不会影响句子的基本意思，通常用逗号与主句隔开。
示例： My brother, who is a doctor, lives in Beijing.
在这个例子中，定语从句"who is a doctor"是对先行词"my brother"的补充说明，去掉后句子的基本意思仍然完整。
注意：非限制性定语从句中不能使用that引导，且关系代词不能省略。
在英语中，不能同时使用两个连词来连接两个句子，这是中式英语中常见的错误。
错误示例：
Because he was sick, so he didn't go to school.
Although it was raining, but we still went out.
正确示例：
Because he was sick, he didn't go to school.
He was sick, so he didn't go to school.
Although it was raining, we still went out.
It was raining, but we still went out.
句子片段是指不完整的句子，通常是一个孤立的从句，没有主句。
错误示例：
Because I want to improve my English.
Which is very important for my future.
正确示例：
I study hard because I want to improve my English.
I am learning English, which is very important for my future.
在使用定语从句时，关系词的选择错误是常见的问题，特别是在who和whom、which和that之间的混淆。
错误示例：
The man which is talking to my teacher is my father.
This is the book what I bought yesterday.
正确示例：
The man who is talking to my teacher is my father.
This is the book that/which I bought yesterday.
悬垂修饰是指修饰语在句子中没有明确的被修饰对象，或者修饰了错误的对象。
错误示例：
After finishing the homework, the TV was turned on.
Walking along the street, a car hit him.
正确示例：
After finishing the homework, he turned on the TV.
While he was walking along the street, a car hit him.
逗号粘连是指使用逗号连接两个独立的句子，而没有使用适当的连词或分号。
错误示例：
I like English, it is very interesting.
She is my best friend, we often help each other.
正确示例：
I like English because it is very interesting.
I like English; it is very interesting.
She is my best friend, and we often help each other.