Here's why:
* Comparative forms: The comparative degree of an adjective compares two things. We usually form the comparative by adding "-er" to the adjective (e.g., taller, faster, older) or using "more" before the adjective (e.g., more beautiful, more interesting, more difficult).
* One-syllable adjectives: One-syllable adjectives typically form the comparative using "-er." Adding "more" in this case is unnecessary and grammatically incorrect.
Example:
* Correct: The red car is faster than the blue car.
* Incorrect: The red car is more fast than the blue car.
Exceptions:
There are a few exceptions where "more" is used with one-syllable adjectives, but these are mainly for emphasis or to avoid awkwardness. For example:
* More common: This can be used to emphasize the frequency.
* More rare: This can be used to avoid the awkwardness of "rarer."
In most cases, stick to the standard "-er" ending for one-syllable adjective comparatives.