Learn Danish: Hun vasker sine kjoler. - She washes her dresses.

sine is a reflexive possessive, meaning it refers back to the subject of the sentence. Because she is washing her own dresses, we use sine. If the sentence said Hun vasker hendes kjoler, it would actually mean she is washing another woman's dresses!

How do I know whether to use sin, sit, or sine here?

The word you choose always depends on the noun that comes right after it. sin is for singular en-words, sit is for singular et-words, and sine is for all plural words. Because kjoler (dresses) is plural, we must use sine.

Why doesn't kjoler have the definite ending attached, like kjolerne?

In Danish, whenever you use a possessive word like sine, min (my), or hendes (her), the noun that follows it must be in the indefinite form. Even though we are talking about specific dresses, the possessive word already does the job of pointing them out, so we just use the base plural kjoler (dresses) instead of kjolerne (the dresses).

Does the verb vasker change if someone else is doing the washing?

No! In Danish, verbs do not change based on who is doing the action. In English we say "I wash" but "she washes", whereas in Danish it is jeg vasker and hun vasker. The present tense stays exactly the same.