Can I say " The girl is more short than the boy " and when can I use more with short adjective?
You can use more with short adjectives if more also modifies an adjective that doesn't take -er, as in more silent and sad. (You can also say more silent and sadder.)
There are also a few one-syllable adjectives that form their comparatives with more – the most common are probably fun, real, wrong, and right. The comparatives funner, realer, wronger, and righter are only used rarely, and many people will perceive them as incorrect, although they will understand them.
But aside from these exceptions, we generally use more only with adjectives that are two syllables and longer, and use -er with one-syllable adjectives.