"Do you know him?"

Translation: Ти знаєш його?

[JanuszWoro3]

Is "ти" necessary ? Can't you just say "знаєш його"?

[Sosnytskiy]

"знаєш його?" is definitely understandable, but for me it feels a bit unusual. Like from somebody who would not waste time on you. But may be it's just me.

[thomaszabel]

I can't speak for Ukrainian, but I'm fluent in Russian. Because of verb endings, my guess is that ти is optional. So you could even say "Його знаєш?" and it would be fine. In fact when i speak with native Russian speakers, that's the preferred conversational way to say it.

An example:

Знаєш?
Знаю.

This would translate to:

Do you know?
Yeah, I know.

Very conversational. Fewer words.