Learn Swedish: De är röda och snabba.

Because the subject of the sentence, de (they), is plural. In Swedish, adjectives must match the subject they describe. Since there is more than one thing being described, we add an a-ending to the base adjectives röd (red) and snabb (fast).

How is the word De pronounced in this sentence?

Even though it is spelled de, native speakers almost always pronounce it like dom in everyday spoken Swedish. You usually only hear it pronounced the way it is spelled in very formal situations.

If I change the sentence to mean 'It is red and fast' for a single car (en bil), do the adjectives change?

Yes! Because a car is a singular en-word, you drop the plural a-endings. The sentence becomes Den är röd och snabb.

Is there a trick to pronouncing the word och?

Yes, in everyday spoken Swedish, the word och (and) is almost always shortened and pronounced simply like the Swedish letter å.