“He is angry,so we --better be carfull,”she said.

A had
had better do sth 固定搭配,意为：最好.
had better(常简略为'd better)是一固定词组,had better"最好",用于表示对别人的劝告、建议或表示一种愿望.其用法有以下几点：
一、had better后面必须跟动词原形.
had better后跟动词原形(即不带to的不定式),构成had better do sth.句型.
这里的had不能用have来替换.如：
You'd better go to hospital at once.你最好立即去医院看病.
Tom,you'd better go there today.汤姆,你最好今天去那儿.
二、主语不论是第几人称,句子不论是什么时态,都要用had better的形式.如：
Now you(he,we) had better listen to the teacher.你(他,我们)现在最好听老师讲.