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"why does my child act aggressive at school?"
"why does my kid act aggressive toward his brother?"

Shouldn't we say "act aggressively" ? Or is it just some kind of shorthand expression from an upset parent?
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Yes, "act aggressively" is correct.

Adverbs of manner describe how an action happens or is done, and end in "ly".

The adverb describes how the child acts, so it ends in "ly".
It would be okay to ask, "Why is he so agressive toward his brother?" In this case the predicate adjective is complement to the being verb.
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Yes that's fine.

Don't forget aggressive has 2 g's.
Hmm, interesting. Isn't "act" a linking verb too in some cases? I think it can be used as a linking verb when it means "pretend", but I am not sure when it means "behave"... It sure is used as a linking verb pretty often in several dialects, anyway, or in informal registers. Stop acting weird!I wouldn't even know another way to say it, since "Stop acting weirdly!" sounds... weird. Emotion: smile
BUt I don't like "act aggressive", so I think I would say "act aggressively" in that case. Maybe it's just me, lol, go figure.
Interesting K, I wouldn't have noticed that.

I think that "act" can have two senses - what are are doing right now (you're acting weird) and an umbrella word to describe your actions (you act aggressively toward your brother).

Kind of like You're being a jerk vs. You are a jerk.

Maybe?
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Grammar GeekI think that "act" can have two senses - what are are doing right now (you're acting weird) and an umbrella word to describe your actions (you act aggressively toward your brother).

Kind of like You're being a jerk vs. You are a jerk.

Maybe?
Yeah, it might be that "act" is used as a linking verb when we want to describe something that appears to be temporary, while "acting aggressively" sounds more general. But as always, English is so unpredictable that I wouldn't be 100% sure. Emotion: wink