大学英语：either

(1)表示两者中的任何一个，用单数动词。

You may take either of the roads to get there.

He could write with either hand.

(2)表示两边。

You can see a lot of trees on either side of the street.

Many conversations are a kind of competition that rarely leads to discovery on either side.

(3)作副词，表示也，用于否定句。

A little bit of exercise won’t do you any harm,either.

He didn’t go to the meeting and she didn’t turn up,either.

(4)和or连用，连接相同的语法结构，表示或是……或是，不是……就是，既……又，既不……又不，它所带的动词在数上与or后面的名词或代词保持一致。

Either you or Dick is responsible for it.

Either he or his brothers have  to clean the room.