Don't act stupid/stupidly.

[diamondcutter]

🆚What is the difference between "Don't act stupid." and "Don't act stupidly." ? "Don't act stupid." vs "Don't act stupidly." ?

Synonym for Don't act stupid. Technically, you need an adverb to modify "act", so "act stupidly" would be correct, but we frequently use a "flat adverb" and say "Don't act stupid". It is better to say "Don't be stupid" so that stupid becomes an adjective. Nobody says "Don't act stupidly". |Don't...

hinative.com

On this web page, a native speaker (?) says this:

Nobody says "Don't act stupidly".

I’d like to know if what he/she says is true.

 

[emsr2d2]

I wouldn't go so far as to say "Nobody says". It's true that it's more common to hear "Don't act stupid". If I had to try and explain it, I'd say that it's a shortened form of "Don't act as if you were stupid".

I'd use the two in different contexts.

Mum: What's one plus one?
Bob: Ummmmmm, seventeen?
Mum: Don't act stupid. You know perfectly well what the answer is.

Mum: What are you doing up there?
Bob: I'm bouncing on the bed to see if I can hit my head on the ceiling.
Mum: Don't act [so] stupidly. You'll hurt yourself.

 

[Tarheel]

I think Bob was trying to be funny.

 

[Tdol]

I’d like to know if what he/she says is true.

It's overstating the case as you can see from these Ngram graphs.