There are two types of (absolute) adjective. When an adjective comes logically "before" a noun, it is attributive.

The happy boy smiles.
The happy boys smile.

When an adjective comes logically "after" the noun and copula (e.g. is, was, etc), they are predicative.

The boy is happy.
The boys are happy.

Note in the above English examples about the happy boy(s), the adjective remains the same whether there are one boy or plural boys.

In Hungarian, attributive adjectives usually remain in the singular, just like in English.

• a boldog fiú mosolyog. the happy boy smiles.
• a boldog fiúk mosolyognak. the happy boys smile.

Note here how when we pluralise the happy boy, the adjective (boldog) remains singular.

a boldog fiú mosolyog.
a boldog fiúk mosolyognak.

Note in the above English examples about the boy(s) being happy, the adjective remains the same whether there are one boy or plural boys.

In Hungarian, predicative adjectives match in number for their noun, unlike in English.

• A fiú boldog. The boy is happy.
• A fiúk boldogok. The boys are happy.

• A lány szép. The girl is beautiful (predicative).
• A lányok szépek. The girls are beautiful (predicative).

• boldog volt a fiú, hogy látja a kutyáját. Happy was the boy that he sees his his dog.
• boldogok voltak a fiúk, hogy látják a kutyájukat. Happy were the boys that they see their dog.

Note here how when we pluralise the boys being happy, the adjective (boldog) now matches the plural of the boys.

A fiú boldog.
A fiúk boldogok.

When we pluralise a sentence that contains both attributive and predicative adjectives, the rules do not break. We pluralise the predicative, and not the attributive.

In this example, "Hungarian" is attributive and "beautiful" is predicative.

• Az a magyar lány szép. That Hungarian girl is beautiful.
• Azok a magyar lányok szépek. Those Hungarian girls are beautiful.

I have used speech marks to indicate that "before" and "after" are meant logically, not lexically. That is, when I say "after" the adjective does not have to be written after the noun on the paper. Consider this:

Happy were the boys to see their dog.

Predicative or attributive? It's written on the paper before the noun, but it applies logically afterwards. Here, happy is predicative.