"The apples are very heavy!"

Translation: Az almák nagyon nehezek!

[NiclasLang]

Can I not use súlyos here?

[vvsey]

"Súlyos" is perhaps a bit too heavy for apples. A heavy burden can be "súlyos", or a serious illness. Or the cargo on your truck. Or a personality. All in all, overly heavy things. But grammatically it is okay. For "súlyos almák" though, I would think they are made of stone.

[96314081311257]

Yes, you could.

[Rachelle73965]

Doesn't "Az nagyon nehezek almák" also work?

[Tim278579]

No. Because the adjective precedes the noun, it doesn't decline for case or number. So "nehéz" is needed, not "nehezek".