到底是pretty bad还是pretty badly?

问题补充：
He speaks English pretty ___badly _______(worst).
虽然意思差不多,但是pretty bad 是形容词性的词组.
这个句子是speaks ,说做为中心词,要用副词形式.
后面我给了许多例句,可以看下.
pretty bad 相当差;
Pretty bad. 糟透了;
That’s a pretty bad state 状况实在不妙;
He is in pretty bad shape now.
他现在状况很糟糕.
Signs are it will be a pretty bad one.
有迹象表明,这将是个很糟糕的残局.
pretty badly 相当坏;
The bad news has shaken her up pretty badly.
那坏消息一直使她感到极度不安.
That motorcycle was his pretty badly.
那辆摩托车被撞得很重.

pretty bad