She speaks English very ...
She speaks English very bad or badly?
which one is correct and why?
what about
Her English is bad or badly?
I need a reason more than the answer.

Question

She speaks English very ...
She speaks English very bad or badly? 
which one is correct and why?
what about
Her English is bad or badly?
I need a reason more than the answer.

Answer

Badly is an adverb. Adverbs describe verbs.
She speaks English badly.

Bad is an adjective. Adjectives describe nouns.
Her English is bad.