"The fat men are not good actors."
Translation:A kövér férfiak nem jó színészek.
Because you don't use the plural form of an adjective if it is before a noun (the noun already shows that there are more than one of them). You use adjectives in plural only if they are the predicate of the sentence.
"Itt vannak a kövér férfiak." = "The fat men are here."
"A férfiak kövérek." = "The men are fat."