both“两者都…”可作代名词、形容词,复数含义,如：
They both like swimming.＝Both of them like swimming.
Both his brothers like swimming.
either 两者中任何一个,单数含义,名词、形容词用法,如：
There are many flowers on either side of the street.
each指许多人或物中的“各个”,侧重于个别.如：Each student should have his own way to study．
either和both均表两者,但前者跟名词单数,而后者跟名词复数,所以会有either side和both sides之分. both“两者都…”可作代名词、形容词,复数含义,如：
They both like swimming.＝Both of them like swimming.
Both his brothers like swimming.
either 两者中任何一个,单数含义,名词、形容词用法,如：
There are many flowers on either side of the street.
each指许多人或物中的“各个”,侧重于个别.如：Each student should have his own way to study．
either和both均表两者,但前者跟名词单数,而后者跟名词复数,所以会有either side和both sides之分.