I see this all over the in place in grammar books. Where did anyone get the idea that ‘the’ is an adjective? This is clearly wrong. Linguists would call it a ‘determiner’, but even the more traditional name of ‘article’ would be fine. So long as it gets put into a different category from words like ‘delicious’ or ‘colorful’, it doesn’t really matter what you call it.
Three simple reasons why the determiner the, and its companion a, should not be considered adjectives:
(1) Adjectives can “stack”, but the determiners do not:
The huge hairy angry black bear.
*The an a bear.
(If you don’t know what that star means, see this post)
(2) Adjectives have comparative forms, the determiners do not:
The red dot. The redder dot. The reddest dot.
The dot. *The-er dot. *The-est dot.
(3) Some adjectives can occur after the noun they modify. No determiners do this:
The visible stars tonight are shown on this map.
Shown on this map are the stars visible tonight.
The footballer kicked a puppy.
**Footballer the kicked puppy a.