Parts of speech are lexico-grammatical classes of words differentiated from each other by

Adjectives and adverbs
Parts of speech are lexico-grammatical classes of words differentiated from each other by:
a) a generalized, categorical meaning, i.e. semantically;
b) specific inflectional and derivational features, i.e. morphologically;
c) syntactic function in a sentence, i.e. syntactically.
Accordingly, grammarians discriminate between notional and functional parts of speech. To the notional parts of speech belong nouns, adjectives, numerals1, pronouns, verbs and adverbs, while functional parts of speech are represented by determiners (articles), prepositions, conjunctions and particles.
Notional parts of speech can be used independently in a sentence performing syntactic functions of a subject, object, predicate, attribute or adverbial modifier while function words can’t perform independently any syntactic function in a sentence. Notional parts of speech can act as heads in phrases and most of them constitute open classes of words (except pronouns and numerals), whereas function words represent closed classes of words, which have a very limited and fixed membership.
Below, we offer the survey of the nominal parts of speech according to the scheme: se-
mantics - morphological features – syntactic function (i.e. meaning – form – function).
adjective (noun): a part-of-speech that modifies or describes a noun or a pronoun.
An adjective is one of the nine parts of speech.
An adjective is a word that tells us more about a noun. It "describes" or "modifies" a noun (The big dog was hungry). In these examples, the adjective is in bold and the noun that it modifies is in italics.
An adjective often comes BEFORE a noun:
a green car
a dark sky
an interesting story
And sometimes an adjective comes AFTER a verb:
My car is green.
The sky became dark.
His story seemed interesting.
But adjectives can also modify pronouns (She is beautiful). Look at these examples:
They were empty.
I thought it seemed strange.
Those are not expensive.
Note that we can often use two or more adjectives together (a beautiful young French lady / it is black and white).
The adjective is the enemy of the noun
This is sometimes said because, very often, if we use the precise noun we don't need an adjective. For example, instead of saying "a large, impressive house" (2 adjectives + 1 noun) we could simply say "a mansion" (1 noun).
Adjective Form
Some adjectives have particular endings, for example:
-able/-ible: washable, credible
-ish/-like: childish, childlike
-ful/-less: careful, careless
-ous: dangerous, harmonious
-y: dirty, pretty
However, many adjectives have no obvious form.
Comparative, Superlative
Most adjectives can be comparative or superlative, for example:
big, bigger, biggest
good, better, best
beautiful, more beautiful, most beautiful
In linguistics, an adjective (abbreviated adj) is a word that modifies a noun or noun phrase or describes its referent. Its semantic role is to change information given by the noun.

Traditionally, adjectives were considered one of the main parts of speech of the English language, although historically they were classed together with nouns.[1] Nowadays, certain words that usually had been classified as adjectives, including the, this, my, etc., typically are classed separately, as determiners.

Some examples:
That's a funny idea. (attributive)
That idea is funny. (predicative)
Tell me something funny. (postpositive)
The good, the bad, and the ugly. (substantive)
Comparative adjectives
We use comparative adjectives to show change or make comparisons:
This car is certainly better, but it's much more expensive.
I'm feeling happier now.
We need a bigger garden.
We use than when we want to compare one thing with another:
She is two years older than me.
New York is much bigger than Boston.
He is a better player than Ronaldo.
France is a bigger country than Britain.
When we want to describe how something or someone changes we can use two comparatives with and:
The balloon got bigger and bigger.
Everything is getting more and more expensive.
Grandfather is looking older and older.
We often use the with comparative adjectives to show that one thing depends on another:
The faster you drive, the more dangerous it is.
(= When you drive faster, it is more dangerous.)
The higher they climbed, the colder it got.
(= When they climbed higher, it got colder
We usually add –er and –est to one-syllable words to make comparatives and superlatives:
old older oldest
long longer longest
If an adjective ends in –e, we add –r or –st:
nice nicer nicest
large larger largest
If an adjective ends in a vowel and a consonant, we double the consonant:
big bigger biggest
fat fatter fattest
If an adjective ends in a consonant and –y, we change –y to –i and add –er or –est:
happy happier happiest
silly sillier silliest
We use more and most to make comparatives and superlatives for most two syllable adjectives and for all adjectives with three or more syllables:
careful more careful most careful
interesting more interesting most interesting
However, with these common two-syllable adjectives, you can either add –er/–r and –est/–st or use more and most:

common
cruel

gentle
handsome
likely narrow
pleasant
polite
simple
stupid
He is certainly handsomer than his brother.
His brother is handsome, but he is more handsome.
She is one of the politest people I have ever met.
She is the most polite person I have ever met.
The adjectives good, bad and far have irregular comparatives and superlatives:
good better best
bad worse worst
far farther/further farthest/furthest