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

I know the rule for making the comparative and superlative form for two-syllable words ending in y, replace the -y with i and use -er and -est:

hap.py → happier → (the) happiest
ti.dy → tidier → (the) tidiest
fun.ny → funnier → (the) funniest

Instead for two-syllable adjectives which do not end in -y, use more and most:

bor.ing → more boring
wor.ried → more worried
care.ful → more careful
tra.gic → more tragic

However, there are inexplicable exceptions: The Free Dictionary says the comparative and superlative form of clever is cleverer and cleverest. Yet to my ears

He's more clever than I thought

sounds more formal and correct.

Google produces a total of 15 pages for “he is more clever than” and 16 pages for the contracted form, “he's more clever than”. Similarly, Google yields 16 pages and 15 pages for “he's cleverer than” and “he is cleverer than” respectively, which suggests there is very little to choose between the two comparative forms.

Meanwhile, TFD insists that the comparative and superlative form of simple is simpler and simplest. Google seems to concur and produces 17 pages for “it is simpler” compared to only 9 pages for “it is more simple”.

The two-syllable adjectives that I am aware of, which have both kinds of comparative and superlative forms are:

• clever → cleverer/more clever → cleverest/(the) most clever
• common → commoner/more common → commonest/(the) most common
• gentle → gentler/more gentle → gentlest/(the) most gentle
• humble → humbler/more humble (etc.)
• hollow → hollower/more hollow
• narrow → narrower/more narrow
• polite → politer/more polite
• quiet → quieter/more quiet
• simple → simpler/more simple
• stupid → stupider/more stupid
• subtle → subtler/more subtle

Etymologically speaking, is there any explanation for this? Is it a recent trend? It seems to me that the number of two-syllable adjectives that add the suffix -er and -est are increasing.

And finally, is there a trick or rule which I can teach my advanced private students? With younger learners and beginners I teach the “rule” that I mentioned at the beginning—so much simpler! :)

EDIT December 11, 2019

I shall never ever understand Google's algorithms and how they produce their statistics. In the end, for the sake of clarity, the number of results have been substituted and updated with the number of pages each search produced.

According to Swan in Pratical English Usage (p114) the two-syllable adjectives whose comparative form is most likely to be formed with -er are those that end with an unstressed vowel; e.g. narrow, simple, clever, subtle, etc. from your list above. Swan goes on to state:

With many two-syllable adjectives (e.g. polite, common) -er/-est and more/most are both possible. With others (including adjectives ending in -ing, -ed, -ful, and -less), only more/most is possible. In general, the structure with more/most is becoming more common. To find out the normal comparative and superlative for a particular two-syllable adjective, check in a good dictionary.

It is interesting that Swan himself uses more common and not commoner in his explanation above, and this seems the better choice in a formal written context. So if you are looking for guidelines for your advanced students I would recommend:

• Learn the common two-syllable adjectives ending with an unstressed vowel that can be compared with -er.
• For the rest use more. I suspect that native speakers are much more likely (likelier?) to consider an -er usage problematic than a more usage. For example, more polite or even more clever will probably sound less ungrammatical than pleasanter or tranquil(l)er.

If your students really would like to know word-by-word if the -er comparative is possible, they will need to consult a good dictionary. Swan recommends the Oxford Advanced Learner's Dictionary, the Cambridge Advanced Learner's Dictionary, the Longman Dictionary of Contemporary English, the MacMillan English Dictionary and the Collins Cobuild English Dictionary.

The Collins, for example, shows pleasanter but more tranquil as the comparative forms.

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.