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To be [linking verb]: to behave in a particular way
He was just being rude.
Don't be silly.
You'd better be careful.
So, we can say "He was just being rude." but why can't we say "He was just behaving rude."
We can only say "He was just behaving rudely."
So, what is the main reason?
The definition of "to be" as a linking verb which you cite ("to behave in a particular way") does not imply that the verbs "to be" and "to behave" can be used interchangeably.
The verb to be is "special" in English, as it is in most languages. To be is complemented (or modified) by an adjective, and not by an adverb, as is the case with most verbs, such as "to behave."
