Questions & Answers about Værelset er stille om aftenen.
The -et is the definite article attached to a neuter noun.
- et værelse = a room
- værelset = the room
In Danish, the definite article is usually a suffix on the noun instead of a separate word like English the.
With parts of the day (morning, evening, night) you usually use om to express “in the …” in a general or habitual sense:
- om morgenen = in the morning / in the mornings
- om aftenen = in the evening / in the evenings
i aftenen is not idiomatic in standard Danish. For a specific evening (tonight) you say i aften, not om aftenen.