Don't be sad VS don't sad

[Polyester]

Don't be sad VS don't sad

Whats the difference between two sentences?

When I omitted the word "be" ...

 

[teechar]

"Don't sad" is impossible on its own. It's possible to say, e.g., "Don't sad stories seem boring?", but that's completely different.

 

[emsr2d2]

"Don't be sad​" [STRIKE]VS[/STRIKE] vs "Don't sad".

What's the difference between those two sentences (no question mark here) [STRIKE]When[/STRIKE] given that I omitted the word "be" in the second? [STRIKE]...[/STRIKE]

See my corrections above.

"Sad" is an adjective not a verb. The imperative "Don't" (as an instruction) must be followed by the bare infinitive of the relevant verb.

 

[Tdol]

And the verb from sad is sadden, though Don't sadden wouldn't work on its own either.