"The small Japanese trees are pretty."

Translation: A kicsi japán fák szépek.

[anton_t13]

"A kis japán fák szépek" is error and the right translation is "a kicsi (...)".
I'm confused. What is the rule?

[vvsey]

Both should be accepted here.

Other than that, here are the differences:

"Kis" can be used in compound words:

• kislány - litle girl
• "kisfiú - little boy

"Kis" can only stand in front of a noun, not behind. Consequently it can only be singular.

Small tree:

• kis fa
• kicsi fa

Small trees:

• kis fák
• kicsi fák

The tree is small:

• a fa kicsi (not possible with "kis")

The trees are small:

• a fák kicsik (not possible with "kis")

[OlgaZubkov]

I think that aranyosak is a more accurate translation for pretty than szépek