从句的分类,
根据主句所缺成分不同总体而分为可以有三大类：定语从句、状语从句、名词性从句
定语从句有两种：限定性和非限定性定语从名
名词性从句有4种：主语、宾语、表语从句，同位语从句
状语从句就多了，什么时间状语，地点状语。原因状语等等等。
想知道怎么判断主句和从句，首先你得理解句子的语素组成。。。主谓宾表状之类的。。。
在一个复合了主句和从句的句子中，你可以根据两个谓语来分开两个句子（一个从句一个主句），
当然也有可能一个句子中包涵多个从句，同理。
分开后可以注意到一个句子是完整的一个是不完整的（名词性从句），或者两个都是完整的（状语从句，状语从句）
如过是名词性从句，不完整的就是主句，同时看看主句不完整在哪（缺什么成分），就可以判断出是什么名词性从句。
两个句子都是完整的就要仔细分析意思了。
例如：
thus
the
arab
umayyed
dynasty
of
caliphs,
which
had
moved
the
capital
from
medine
to
damascus
in
661,
came
to
be
regarded
with
much
justification
as
a
parasitic
clique
that
had
outlived
its
usefulness
once
the
conquests
were
completed.
人们开始理由充分地认为，阿拉伯伍麦叶里发王朝是征服战争完成后毫无作用的一个寄生集团；伍麦叶王朝曾于661年从麦地那迁都大马士革。
逗号后面的是非限制性定语从句，直接分离得到
1）which
had
moved
the
capital
from
medine
to
damascus
in
661；
2）thus
the
arab
umayyed
dynasty
of
caliphs
came
to
be
regarded
with
much
justification
as
a
parasitic
clique
that
had
outlived
its
usefulness
once
the
conquests
were
completed.
2)句子中出现的谓语
有
came
to
be
regarded
和
had
outlived
和
were
completed
从而分离
a）thus
the
arab
umayyed
dynasty
of
caliphs
came
to
be
regarded
with
much
justification
as
a
parasitic
clique
（主句）
b）that
had
outlived
its
usefulness
（定语从句）
c）once
the
conquests
were
completed（条件状语从句）
——————————————————————————————
总的来说，可能我提及的有点复杂了，
你现在学习的是什么类型的从句呢，欢迎追问，我仔细说说。
同时这只是自己的一些学习经验，学的时间久了可能有错误的存在，
如过有哪些错误的地方，欢迎交流斧正。
谢谢。
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