如果指在光秃秃的山上,用on,如果强调在树丛中 用in ,这要看语境的
You can ski on the hills.
你能在山上滑雪.
Mr Jones:"Walk on the hills." OK?
琼斯先生：“在山上漫步”可以吧
The creek originates in a spring in the hills.
小河源自山泉.
The kidnappers' lair was an old farm in the hills.
绑架者的藏身处是山里的一座旧农场.
He earned his living cutting firewood in the hills .
他靠在山上砍柴为生.