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!