墙头草的英文：
grass on the top of a wall which sways with every wind; a person who bends with the wind

参考例句：

How can you trust him? Don't you know that he always sits on the fence and falls on the side of the stronger?
你怎么能够相信他呢?他是个墙头草,谁硬就跟谁。

grass是什么意思：
n. 草；草地
v. （使）长草；揭发

Grass Plant Taxonomy
禾本科植物分类学

There is melancholy in the wind and sorrow in the grass
风儿忧郁，草儿悲哀

The grass in the yard was waist high.
院子里的草有齐腰高。

wall是什么意思：
n. 墙，壁；人墙；围墙；屏障；障碍
v. 用墙围住；用墙隔开；禁闭

There is a blank wall.
有面无窗的墙。

The walls are beige.
墙壁都是米色的。

This is a plumb wall.
这是一面垂直的墙。

which是什么意思：
pron. 哪一个，哪些
adj. 哪一个，哪一些

Can you tell which is which?
你能看得出哪是哪吗？

Which side are you on?
你属于哪一派？

I wonder which party, which faction, you are with!
我就怀疑，你这个人是那一党那一派！