The rules
* 1 syllable: -er than / the -est
example: strange - stranger than - the strangest
* 2 syllables ending in -le, -er, -ow, -y, -some: -er than / the -est
example:
simple - simpler than - the simplest
* 2 syllables not ending in -le, -er, -ow, -y: more /most
example: careful - more careful than - the most careful
3 or more syllables: more ... than / the most
example: popular - more popular than - the most popular