"To jest zegarek Adama."

Translation: This is Adam's watch.

[Jack.Elliot]

this is the watch of Adam

[karlr42]

..is not a natural English sentence.

If you are going to put the words in that order, I would say "This watch is Adam's".

[MetalCotta]

I know one Adam from Poland. He's cool person, and because of him I'm here!

[alik1989]

Good for you! :)

[Titan4eg]

"This is Adam's handwatch"?

[Jellei]

I don't believe that "handwatch" is commonly used in English. "wristwatch" is, though.

[Ryszard386408]

What is wrong with "This watch is Adam's"?

[Jellei]

Mostly the fact that the subject of your sentence is completely different ("this watch" rather than "this/that/it").