To compare, add -mpi to the adjective — the job of English -er and more. Than is kuin:

• Tämä paita on halvempi kuin tuo. — This shirt is cheaper than that one.
• Helsinki on isompi kuin Turku. — Helsinki is bigger than Turku.

How the stem behaves:

Base	Comparative	Rule of thumb
iso (big)	isompi	most words: just add -mpi
helppo (easy)	helpompi	gradation: pp → p
vanha (old)	vanhempi	two syllables ending in -a / -ä → -empi
halpa (cheap)	halvempi	…and gradation: p → v
pitkä (long, tall)	pidempi	…and gradation: t → d
kallis (expensive)	kalliimpi	-is → -iimpi
kaunis (beautiful)	kauniimpi	-is → -iimpi
pieni (small)	pienempi	-i words often → -empi