When we want to compare two things, we add –er.
When the word has only three letter (consonant + vowel + consonant), we double the last letter.
simpler / more simple
happier / more happy
When the word has two syllables, we can use –er or we can use more. When the word has two syllables and it ends with –y, we change –y into –ier.
When the word has three syllables, we use more.
good – better
bad – worse
Some words use special forms.
The wind is stronger. The ship is faster. The clouds are bigger.
You can see more examples in the book Robinson Crusoe.