"The telephones are not light."

Translation: A telefonok nem könnyűek.

[Andrew359786]

Why könnyűek rather than simply könnyűk?

[RyagonIV]

Adjectives sometimes include a binding vowel with their -k suffix, even though the base word already ends with a vowel. Point in case: férfiak.

Might be easier to just note them down as exceptions, though.