The fall of one leaf is enough to tell coming of autumn. 

一叶知秋。

In autumn,leaves turn yellow.

秋天树叶变黄。

In autumn the leaves fall down from the trees.

秋天树叶从树上掉下来。

American people use the word "fall" to mean autumn.

美国人用"fall"这个字来表示秋天。

I like Beijing particularly in autumn.

我爱北京，特别是在秋天。

There is a beauty called autumn in Mount Huangshan.

有一种美，叫黄山的秋天。