The comparative form of most monosyllable (1 syllable) and disyllable (2 syllable) adjectives and adverbs is made by adding –er to the adjective.
I am big, he is bigger.
She works fast, he works faster.
The comparative form of trisyllable (3 syllables) and polysyllable (more than 3 syllables) adjectives and adverbs is made by using more (or less) before the adjective or adverb.
I don’t know a more influential man in town.
She should dress more tastefully.
(See below for exceptions to this rule.)
The second term of comparison (when there is one) is introduced by the subordinating conjunction than followed by the item the first one is being compared to.
[comparison] + than + [second item]
He is taller than me.
We are more efficient than a machine.
She is prettier than her mother.
The shops in this neighborhood are less fashionable than those in the city center.
More or -er?
As mentioned above, in general we add -er to 1 or 2 syllable words and more to 3 or more syllable words. However, this rule is often broken and we can see plenty of examples. This table below shows a comparison concordance for the two terms gentle and more gentle.
Whilst in most cases gentler (in brown) is much more common than more gentle (in green) you can see that in a significant number of cases the non-standard more gentle is used; overall through the 10 corpora used more gentle is used in approximately 25% of cases.