This is the 848th most frequent Hungarian word.
der
In Hungarian, "der" has no meaning; it might be a typo or a misinterpretation. In German, "der" is the masculine definite article meaning "the."
Here, 'Derült' is the past participle form meaning 'cleared up'. It is used here as an adjective to describe the state of the sky.
Derült az ég, és csillagok ragyognak.
The sky is clear, and stars are shining.
In this case, 'Derűs' is an adjective derived from 'derű' (cheerfulness), denoting a positive and bright characteristic.
Derűs emberek könnyebben veszik az akadályokat.
Cheerful people overcome obstacles more easily.
Here, 'derülte' refers to the nominalized form of 'derül', indicating the act or event of clearing up related to the weather.