There are trees on both sides of the road.
There are trees on either side of the road.
主要看后面给的名词是单数还是复数~就很容易判断了
Either of the apple has a hole.
Both of the apples have holes.为什么either用单数？either表示“每一+量词”，你的句子中是路的每一边都有树，路的一边用单数both of 是两者的意思，当然用复数咯～～