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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

And I would say that 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 an impressive 48,000 results for "he is more clever than" and 3,980 results for the contracted form, "he's more clever than" compared to only 6,710 results and 19,800 results for "he's cleverer than" and "he is cleverer than", which suggests that more clever is preferred.

Similarly, TFD insists that the comparative and superlative form of simple is simpler and simplest.
Google seems to concur and produces 331,000 results for "It is simpler than" compared to 110,000 results for "It is more simple than".

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
  

Is there any explanation for this? Etymologically speaking, 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 Feb 9 2015

I have had to update Google figures at least twice now. I shall never understand its algorithms, and how they reach certain number of results. So I apologize if the links posted above produce different figures next week, it is not my fault!

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.

The Oxford English Dictionary provides its answer here.

I personally prefer the rule: For 2 syllables use -er/-est unless that form is not allowed (see OED).

Okay so the simplest answer I can give is, the usage of more clever versus cleverer is relative to two things; third person versus first person, and if the term is referencing a person or thing.

For example: (From a narrative third person) Margie was more clever than Dick.
(From a narrative first person) I was cleverer than Dick.

(From a narrative third person) "The engine was most clever" said Margie
(From a narrative first person) "I say the cleverer engine best all the other inventions".

Cleverer however has fallen out of use in modern English, except in jibe and glib commentary. Idioms from the Mid-modern English era post-Shakespeare use cleverer quite often.