mountain的意思是“山”、“山岳”，其复数形式mountains 常表示 “山脉”，通常指比hill 大或陡峭的高山，例如：It’s the highest mountain in the world（它是世界上最高的山）。hill的意思是“小山”、“丘岭”，通常指比 mountain 小的山，例如：The house stands on the top of the hill（房子位于山顶上）。
A mountain is a very high area of land with steep sides.
If you talk about a mountain of something, or mountains of something, you are emphasizing that there is a large amount of it.
If you say that someone has a mountain to climb, you mean that it will be difficult for them to achieve what they want to achieve.
A hill is an area of land that is higher than the land that surrounds it.
We trudged up the hill to the stadium.
If you say that someone is over the hill, you are saying rudely that they are old and no longer fit, attractive, or capable of doing useful work.