Hi Joey

1. er and est is added to nearly all monosylabic adjectives:
He is older than me/I.

2. Many disyllabic adjectives behave in the same way, especially those that end in y, ow, er or a consonant + le:
We are happier than them/they.
This is the narrowest street.
Tom is cleverer than me/I.
He asked a simpler question.

You can choose freely in many cases: commoner/more common, oftener/more often, handsomer/more handsome etc.
There is considerable variety and a tendency to use more and most even with short adjectives, especially when emphasis is needed. Very few will argue with you unless you say or write something totally exceptional: I'm tireder than you.

3. Adjectives longer than two syllables form their comparatives and superlatives with more and most:
This is the most interesting book.
Is German more difficult than Arabic?

4. Adverbs ending in ly always take more and most:
He drove more slowly than us/we.

Cheers
CB