Sadly, there is no perfect rule. Here are some general rules: 1. If an adjective has one syllable, we usually use -er: rich / richer. 2. Some two-syllable words also use -er: thrifty [careful with spending money] / thriftier. 3. If an adjective has three syllables, we usually use "more": beautiful / more beautiful. 4. Also some two-syllable words can take "more": candid [very honest]; more candid. 5. Finally, there are some adjectives that can take "-er" or "more": polite / politer; polite / more polite. 6. I suggest that you do two things: a. Check a good dictionary. b. Check the Web for lists of comparative adjectives. 7. Please remember: If a writer can use "-er" or "more," h/she will often make a choice based on sound. Some people think that "You are more handsome than I am" sounds smoother than "You are handsomer than I am." And, of course, some writers think that "handsomer" is better! * I wish to credit a book called English Review Grammar by Walter Kay Smart.

Using "more" will always communicate the meaning, but it sounds strange for most one-syllable adjectives when forming comparative degree. A word like "formal", with two syllables, is borderline. I mean, it is not correct to use "formaler", but other two-syllable adjectives do use that structure, like "dirty" or "sunny". In case of three syllables or ending in "-ous", use "more".

The comparitave for formal is "more formal" kind regards Patrick