Other adjectives with two or more syllables:
Other 2-syllable adjectives: use more with the unchanged adjective
The shops are always more crowded just before Christmas.
Is there anything more boring than reading about grammar?
My sister is more careful with her writing than I am with mine.
Exceptions
The comparative of some shorter 2-syllable adjectives can be formed with -er.
To be sure which comparative method to use, you will need to consult a good dictionary.
Adjectives with three syllables or more:
Adjectives with 3 or more syllables: use more and unchanged adjective
Russian grammar is more difficult than English grammar.
My sister is much more intelligent than me.
I find maths lessons more enjoyable than science lessons.