"Jánosnak szép kertje van."

Translation: János has a beautiful garden.

[SedatKlc]

to which last consonants do we have to put ja,je ? is there a rule for it?

[jsiehler]

Short answer: No, there is not a rule for which 3rd person ending a noun will take if it ends in a consonant. You have to memorize it as part of learning the noun.

There's a little more discussion here:
https://www.duolingo.com/comment/18281289

I think after a while though you will develop a sense for it, and if you get it wrong, it doesn't make a big difference in practice. It's something that even native speakers occasionally have to look up or ask about.

[SedatKlc]

how can i check which suffix that it takes?

[jsiehler]

A good dictionary will show the 3rd person possessive form.

The only one that I know online that provides this is Wiktionary.

For example, kert takes -je

https://en.wiktionary.org/wiki/kert

While világ takes -a

https://en.wiktionary.org/wiki/vil%C3%A1g#Hungarian

[SedatKlc]

thanks a lot

[RyagonIV]

You can also look into dict.cc. Might be a bit easier to handle. Right above the entries on the left side, it lists the noun's base form, accusative, plural, and 3rd-person singular possessive form.
Wiktionary is a lot more comprehensive, though, if you want to look up special forms.

[RobertEddy]

Good dictionary discussion here.