The comparative form of an adjective is made by adding the -er ending.

Dat huis is groter dan dit – That house is larger than this one.
Zijn tuin is mooier dan de mijne – His garden is more beautiful than mine.

Mind the spelling:
groot → groter
dik → dikker

-er is added to all the adjectives, regardless of the number of syllables:
Die film is interessanter dan deze – That film is more interesting than this one

If an adjective ends in -r, then -der is used to form the comparative form:

lekker → lekkerder
duur → duurder
mager (thin, slim) → magerder
ver (far) → verder

Comparative adjectives add the ending -e according to the same rules as ordinary adjectives when they consist of two syllables:

Wij hebben en grotere tuin nodig – We need a bigger garden.
Zij hebben een kleiner huis gekocht – They have bought a smaller house.

Notice: kleiner does not add -e as it precedes a neuter singular noun with een.

Comparative adjectives of 3 or more syllables do not add -e:

Ik heb een interessanter film gezien – I have seen a more interesting film.