1、表示“反对”“对着”“违反”等。

We are all against his idea. 

我们都反对他的想法。　　

He was married against his will. 

他结婚是有违本意的。

此时，against反义词是for。

Are you for or against the plan?　

这个计划你是赞成还是反对呢?　　

2、表示“靠着”“顶着”“迎着”等。　

He stood with his back against the door. 

他背靠门站着。　　

Bright red flags flow in the wind against the blue sky. 

鲜艳的红旗映着蓝天迎风飘扬。　　

3、against是介词，后接动词时要用动名词。

I’m against doing anything till the police arrive.　　

我反对在警察到达之前采取任何行动。