Rule 1. When two items/people are compared, a comparative degree is used by putting ‘er’ to the adjective word in association with the word ‘than’. In some cases ‘more’ is used.
Comparative degree example:
She is smarter than her sister.
She is more cheerful than her sister.
Similarly, when more than two things/people are compared, the superlative degree is used by putting ‘est’ to the adjective word or in some cases ‘most’ is used.
Superlative degree of comparison examples:
He is the strongest wrestler.
He is the most handsome actor.

Rule 2. ‘More’ is used when you compare qualities of a single thing/person. Even if the first adjective is a single syllable word.
Degree of comparison examples:
Incorrect – She is smarter than clever.
Correct – She is more smart than clever.