In order to compare two or more things you use Adjectives in their comparative or superlative form.
Comparative Adjectives
When an Adjective is short (one syllable), you make the comparative form by putting -(e)r on the end. You put -er when an Adjective ends with a consonant; you put -r when an Adjective ends with a vowel:
long + -er = longer
nice + -r = nicer
Adjectives ending with a single vowel before a consonant double the consonant before adding -er:
fat + -er = fatter
When an Adjective ends with a consonant and -y, you add -er and change the -y into -i:
pretty + -er = prettier
When an Adjective is long (three syllables or more), you put more in front of it:
fascinating + more = more fascinating
Adjectives of two syllables follow one or the other rule. Two-syllable Adjectives ending with -ful, -ing, or -re usually take more:
careful + more = more careful
Two-syllable Adjectives ending with -er, -ow, -le, or -y usually take -er:
clever + -er = cleverer
When you compare two things, you can use the word than:
Peter is tall but John is taller. Peter is taller than John.
Superlative Adjectives
When an Adjective is short (one syllable), you make the superlative form by putting -(e)st on the end. You put -est when an Adjective ends with a consonant; you put -st when an Adjective ends with a vowel:
short + -est = shortest
fine + -st = finest
Adjectives ending with a single vowel before a consonant double the consonant before adding -est:
big + -est = biggest
When an Adjective ends with a consonant and -y, you add -est and change the -y into -i:
happy + -est = happiest
When an Adjective is long (three syllables or more), you put most in front of it:
interesting + most = most interesting
Adjectives of two syllables follow one or the other rule. Two-syllable Adjectives ending with -ful, -ing, or -re usually take most:
unsure + most = most unsure
Two-syllable Adjectives ending with -er, -ow, -le, or -y usually take -est:
shallow + -est = shallowest
The superlative form of Adjectives is used to compare more than two things:
There are three good teachers I know: Mr Smith, Ms Brown, and Mr Keynes. I think Ms Brown is the best teacher.
The superlative form is usually preceded by the:
This novel is the most interesting book I have ever read.
Some (two-syllable) Adjectives can follow both rules for making comparatives and superlatives, though the form with more/most is used more frequently:
polite politer/more polite politest/most polite
pleasant pleasanter/more pleasant pleasantest/most pleasant
stupid stupider/more stupid stupidest/most stupid
common commoner/more common commonest/most common
Note! There are some Adjectives with special, irregular comparative and superlative forms:
good better best
bad worse worst
far farther/further farthest/furthest
many/much more most