What is the difference between types and forms of adjectives?
Comparison of adjectives:
Positive degree – the simplest form.
Comparative degree – comparing two things or persons.
Superlative degree – comparing three or more things or persons.

Positive Comparative Superlative
large larger largest
horrible more horrible most horrible

How to compare adjectives
1. Add er and est to short adjectives of one syllable (sometimes two syllables)
Positive Comparative Superlative
short shorter shortest
great greater greatest
small smaller smallest
old older oldest

2. If a short adjective ends with consonant and y, change the y to i and add er and est.
Positive Comparative Superlative
lucky luckier luckiest
heavy heavier heaviest
happy happier happiest
silly sillier silliest
ugly uglier ugliest

3. If a short adjective ends with e, add er and st.
Positive Comparative Superlative
little littler littlest
fine finer finest

4. Put more(less) and most(least) in front of longer adjectives (with two or more syllables) to show comparison.
Positive Comparative Superlative
appealing less appealing least appealing
remarkable more remarkable most remarkable
beautiful more beautiful most beautiful
alert more alert most alert

If you are uncertain whether to add er and est or to use more and most, then check dictionary. In most cases, you can trust your ears.

Some adjectives are irregular adjectives and don't follow thee rules. They change form considerably from one degree to the next. You need to study these changes carefully in order to recognize them easily.
List of irregular adjectives.
Positive Comparative Superlative
bad worse worst
far further furthest
far farther farthest
good better best
many more most
little less least
old elder eldest
well (healthy) better best

Never use er or est at the same time you use more or most. These are incorrect: more shorter, most alertest

Incomparable adjectives.
Some adjectives cannot be compared because the positive (simplest) form expresses the only degree possible.
List of incomparable adjectives: perfect, unique, fatal, universal, dead, wrong, straight, blind, final, vertical, right, left