Learn English – Is it incorrect to use “more gentle”

The basic rules of forming comparatives:

One-syllable words take "er":

• clear -> clearer
• sweet -> sweeter

Multisyllable words take "more":

• incredible -> more incredible (not "incredibler")
• horrible -> more horrible (not "horribler")

Two-syllable words ending in consonant + "y" take "ier":

• happy -> happier
• pretty -> prettier

Both "more clear" and "clearer" are acceptable:

Your answer is more clear than mine.

Your answer is clearer than mine.

Frequency of use: clearer than is twice as common as more clear than, although both are common.

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.