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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?

  • 1
    "He was just behaving rude" is ungrammatical because "behave" does not take predicative complements like "be" does. Instead it requires a manner complement, typically an AdvP like "rudely", "appallingly", "badly", and the like.
    – BillJ
    Jul 16, 2016 at 7:42

1 Answer 1


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."

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