The general rule is that if an adjective has one or two syllables, you can add -er/-est to the end. If it has three or more syllables, you use more/most + adjective.
hard (1 syllable) > harder/hardest
easy (2 syllables) > easier/easiest
arduous (3 syllables) > more/most arduous
professional (4 syllables) > more/most professional
Bear in mind, though, that it really is a general rule. There are, of course, exceptions.