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.