In Swedish, adjectives should be agreed with the nouns which they predicate. Swedish nouns are divided: en-ord or ett-ord.

For example: en röd lägenhet (a red appartment): because längenhet is an en-ord, so they adjective should be "röd" form. Whereas, if a noun is an ett-ord, for example, ett hus, "röd" will change into "rött": ett rött hus (a red house)

When nouns are plural forms or definitve form, adjectives, again, change their forms. For example, några röda lägenheter (som red apparments) or flera röda hus (several rad houses). Or min röda lägenhet/mina röda lägenheter/mit röda hus/mina röda hus. 

You can see a link here as well http://blogs.transparent.com/swedish/a-little-bit-about-adjectives/

Mr. Tran has it right.
I may add, that the "en/ett phenomenon" is actually not linguistically ruled. It is irregular. However, there are trends. But unfortunately, it is often essential to be a native to recognize them.