"The red carpet"
Translation:Den röda mattan
When an adjective is used an part of a definite for identification, it takes the same form as the plural (eg, "the red carpet" becomes Den röda mattan), based on my understanding of the lessons. Note that in the sentence I used as an example, the word den means "the," not "it." This does not apply when used in the indefinite, so it would still be en röd matta.
When a definite noun is described by an adjective using är, it takes the other form according to its gender, so "the carpet is red" becomes Mattan är röd.
Also, I think a few adjectives are irregular and do not decline in the ways you might expect from what I've just said, but I can't think of any off the top of my head.
I welcome correction if I've made a mistake.