each

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each用作代词时,在句中可作主语、宾语和同位语。
each用作主语时,常用于each of短语,如果each强调的是“个别”,谓语动词与相关的物主代词通常总用单数形式,但如果each强调的是“集体”或先行词是复数或先行词的主语是由两个或两个以上的单数名词或两个复数名词组成时,谓语动词和相关的物主代词则用复数。
each用作复数的同位语时,谓语动词和相关的物主代词也用复数(包括附加的疑问句)。
each other通常只能用作宾语,且总是同复数名词或代词相呼应,表示“互相”这一概念,含有“有来有往”的意味,所以不与conflict, compete等动词连用,且用了each other则不可再加both。
each other的所有格为each other's,其后只接复数名词。

There were three students who called and I gave a book to each.
来了三个学生,我发给每个人一本书。

If you refer to each thing or each person in a group, you are referring to every member of the group and considering them as individuals.
Each is also a pronoun.
Each is also an emphasizing pronoun.
Each is also an adverb.
Each is also a quantifier.
If you refer to each one of the members of a group, you are emphasizing that something applies to every one of them.
You can refer to each and every member of a group to emphasize that you mean all the members of that group.
You use each other when you are saying that each member of a group does something to the others or has a particular connection with the others.