"The dog is dirty."

Translation: Hunden är smutsig.

[Sarah.K.Ha]

I'm confused by some of the rules for adjectives. In the instructions, it says that when the noun is definite, the adjective always takes the "a" ending. Therefore, the translation of "the dog is dirty" should be "hunden är smutsiga", should it not? Am I misunderstanding something, is this an exception to the rule, or a mistake?

[Arnauti]

There are different rules for the attributive position (like in a dirty dog), and the predicative position (like in the dog is dirty).

Compare:
Attributive (an Y X)
en röd bil - a red car
ett rött äpple - a red apple
den röda bilen - the red car
det röda äpplet -the red apples
två röda äpplen/bilar - two red cars

Predicative (X is Y)
Bilen är röd - The car is red
Äpplet är rött - The apple is red
Bilarna/Äpplena är röda - The cars/the apples are red

Edit: cross-posted comment, but I'm leaving it here, hope it can be helpful.

[Sarah.K.Ha]

Thank you to both of you! This clears up my confusion completely.

[beagoodone]

the Best reply ever

[christosam2]

Tack för allt

[Blehg]

The adjective isn't definite in the above sentence. Definite adjectives only appear before the noun that they modify, and unless part of proper names, they are always preceded by a definite article/determiner or a possessive:

den stora staden = the big city
mitt röda hus = My red house
Gustavs nya hund = Gustav's new dog

[beagoodone]

tack så mycket