You are talking about the comparative degree of adjectives. Monosyllabic adjectives and a few disyllabic adjectives are directly followed by -er, such as cold, cold. Very good, even better.

In adjectives ending with stressed closed syllables, when the ending is a vowel letter plus a consonant letter, the ending consonant letter should be doubled and then -er should be added. For example: fat, fat. Sad, sad. Thin, thinner.