1 Syllabe

Cold-Colder-The coldest

Large-larger-the largest

hot- hotter -the hottest

dry-drier-the driest 

2 syllabes

happy-happier-the happiest

busy-busier- the busiest 

2+ syllabes 

modern- more modern- the most modern

intelligent- more intelligent- the most intelligent

good- better- the best 

bad-worse- the worst