etymology - Conundrum: "cleverer" or "more clever", "simpler" or "more simple" etc

The rule that mono- and disyllabic adjectives form their comparatives with -er and larger adjectives do so with more is more or less consistently correct (unless they be participles); however, if you want a more detailed explanation, I have given one below:

There is no absolute rule, but the general trend is that any word that comes from Latin or French into English will form a comparative with more, whereas adjectives of Germanic origin tend to use -er. Past and present participles also use more to form comparatives.

Obvious Latin adjectives usually end in -ive or -ous, both of which form their comparatives with more. E.g. he is lecherous and he is more lecherous, but never he is lecherouser; he seems pensive and he seems more pensive, but never he seems pensiver.

The most notable exception to this is when a Latin comparative or superlative wriggles its way into English; such examples include major 'lit. greater (comparative of magnus 'great') and supreme 'lit. highest (superlative of superus 'high'). These, however, are still not formed regularly by English standards.

Another exception occasionally occurs when adjectives come through French or Vulgar Latin, such as certain, which has in the past formed its comparative as certainer, but this use is almost completely gone. More persistent comparatives of this category include nobler and gentler, largely because of the ease in reducing gentilis to gentle and so on.

French adjectives tend to end in -ant or -ent. Again, a man can be defiant or more defiant, but he cannot be defianter. These adjectives are past participles, and so this rule is also in line with the English rule.

Germanic adjectives, however, almost invariably form their comparatives with -er. Happier, sillier, darker, et cetera are all in this class.

Participles, whether they be Germanic or Latin, present or past, never use -er: he is more annoying, his beard was itching more, the sheep is more shorn, the passage was read more, and so on.

Basically, if it sounds Germanic and is not a participle, use -er; if it sounds Latin, use more.