Questions & Answers about Osten er mild.
Because mild is used predicatively (it follows the verb er). Predicative adjectives in Norwegian remain in their base form regardless of gender or number. Attributive adjectives (before a noun) change:
• en mild ost (“a mild cheese”)
• et mildt brød (“a mild bread,” neuter noun)
• den milde osten (“the mild cheese,” definite form adds -e)
Ost is a common-gender noun (in Bokmål often called masculine/feminine merged). Common-gender nouns take en in the indefinite singular and -en in the definite singular:
• indefinite singular: en ost
• definite singular: osten