Learn Danish: En varieret kost er god for kroppen. - A varied diet is good for the body.

Not naturally. In Danish, en kost is used for your everyday food intake or general nutrition. En diæt typically refers to a restrictive regimen, such as a medical or weight-loss diet. Since this sentence is about general healthy eating habits, kost is the correct word.

Why does the adjective varieret end in a -t?

Varieret is the past participle of the verb at variere (to vary), acting here as an adjective. It ends in -t because that is the standard participle form. It does not change to match the common-gender noun en kost, but if the noun were plural, it would take an -e and become varierede.

Why doesn't the adjective god have a -t at the end here?

Because it modifies en kost, which is a common-gender noun (en-word). You only add a -t (godt) if the adjective describes a neuter noun (et-word), like et måltid (a meal), or if it is being used as an adverb.

Why do we use the preposition for instead of til in this sentence?

When expressing that something is beneficial to your physical health or well-being, Danish uses god for (good for), similar to English. You would use god til if you were describing someone being "good at" a specific skill, like a sport or a language.