we can't see any trees because some people cut them down.为什么用cut them down而不是cutdown them?
cut down 砍到,削减
是由动词cut和副词down构成的短语。
这类短语,当动词后接的人称代词宾格时,只能放在两词之间,不能放在后面。如果cut后的宾语是名词就放在后面。如:cut down a tree.
类似短语还有很多:
put on(put it on)穿上
take off (take them off)脱下