The comparative

Words that only have a syllable or are very short are added an "er" at the end. Example

• Cold-colder
• Long-longer
• Young-younger
• Old-older

If the words have one "y" in the end this is replaced by an "i" and it is added to.

• Heavy-heavier
• Happy-happier
• Pretty-prettier

When the words end in vowel and consonant the same consonant is added and then "er"

• Hot-hotter
• Small-smaller
• Big-bigger
• Thin-thinner

But if the word has more syllables or is larger, it is left the same and the word more is to include

• More expensive
• More exciting  
• More boring

exersise: https://www.perfect-english-grammar.com/comparative-adjectives-exercise-1.html