"Din kostym är perfekt."

Translation: Your suit is perfect.

[Kierat]

We have learned that kostym is a suit, but can be used for costume also? Like a halloween one?

[Lundgren8]

Yes, although dräkt is more common then.

[Liggliluff]

I would say that 'kostym' is a common mistranslation of 'costume'.
"It's recommended that your wear a 'kostym' to a job interview; but don't put on a 'costume'."

[BlazeCyndaquil]

So, we use the '-a' form of an adjective with possessive nouns; but not here?
Is 'perfekt' irregular, or is that only when you use the adjective directly next to a possessive? What is 'perfekt' matching exactly?

[devalanteriel]

Swedish uses both attributive and predicative adjectival forms. What this means is that if the adjective comes before the noun, you use the definite form - and if it comes after the noun, you use the indefinite. Think of it as "the silly jester" versus "the jester is silly" - in English, there's no difference in the adjective, but Swedish frequently has different forms for the two.

[Liggliluff]

No, -a is added to definite and plural nouns.

[Liggliluff]

She pronounces din as [dɨn] ...

[devalanteriel]

Appears to be correct now. :)