When we want to compare two or more nouns using adjectives, we use the comparative and superlative forms of the adjective to show the comparison between the nouns.  

E.g. - 

Honey is sweet, sugar is sweeter but victory is the sweetest. 

In this sentence, we are comparing the three nouns using the positive, comparative and superlative forms of the word ‘sweet’.

Positive Form -  

These are the simple adjectives that simply describe the noun without comparing it to another - big, sweet, clean, etc.  

She has a big black dog.

He is a sweet boy.

The cupboard is clean.

Comparative Form -

These are used when we are comparing two nouns and need to show which noun possesses the adjective or character in a greater or lesser amount, when compared with the other.  - bigger, sweeter, cleaner, etc.

I have a big dog but hers is bigger.

He is sweeter than the other boys.

The cupboard is cleaner than before.

Superlative Form -

 This form is used when three or more nouns are being compared and we need to show that one or more of the nouns posses the adjective or characteristic to the highest amount possible. We usually add ‘the’ before the superlative form. - biggest, sweetest, cleanest, etc.

She has the biggest dog in the colony.

He is the sweetest boy in his class.

The cupboard is the cleanest thing in the house.

 Making Comparatives and Superlatives

There are certain rules that must be followed in the making of the comparatives and superlatives of the adjectives. Not all adjectives form their comparatives and superlatives in the same way and there are also some irregular adjectives that form completely different comparative and superlative forms.

Single Syllable Words and Double Syllable Words ending with -y, -er, -ow, -le -

We use ‘-er’ to make the comparative and ‘-est’ to make the superlative. 

When there is a silent ‘e’ at the end of the positive form, we remove that and add ‘-er’ and ‘-est’ 

When the adjective ends with a ‘y’, we convert the ‘y’ into ‘i’ before adding ‘-er’ and ‘-est’

If the adjective is a small one with little stress on the vowel, we double the last consonant.

Other Words with Two or More Syllables -

For  other double syllable words that do not end with -y, -er, -ow, -le, and for adjectives with more than two syllables we use more and most to form the comparatives and superlatives.

Special Adjectives -

There a few adjectives that can use both ‘-er and -est’ and ‘more’ and ‘most’ to form their comparative and superlative forms. The distinction between these is that ‘-er and -est’ are used when we are comparing the noun to another noun and ‘more’ and ‘most’ is used when we are comparing characteristics within the noun. 

Irregular Comparisons -

These adjectives do not make their comparative and superlative forms using the rules above. Their comparative and superlative forms are different words altogether.

Source: englishleap