What is the adjective in the sentences the red truck was parked behind the garage?

Adjectives describe people, places or things. In the given sentence only two Words describe people or places or things. The phrase "behind the garage" indicates where the truck was parked. That makes it a adverbial. The verbial "was parked" is a past perfect form of a verb. The Word truck is the subject ( a simple noun) of the sentence. That leaves the Words the red. Both the and red describe the subject truck. (Which truck? The red one.) Red is a common adjective describing color. The is a determiner, which arguably is a form of an adjective. But unless you are in college, the answer your teacher wants is red.