Single-Word Requests – Exact Adjective for ‘Conundrum’

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.

• Q1: In the technical language of mathematics, 'soluble' is interchangable with 'solvable', the former is British usage, the latter American. In chemistry, 'soluble' means 'can be dissolved in'.

  'exact' is an adjective modifying 'model', 'exactly' is an adverb modifying 'solvable'. So mathematically these are two distinct things (with the assumption that 'exact' has a stipulated mathematical definition. In informal English, these are hardly distinguishable, especially since it is becoming more common nowadays to drop the '-ly' (or put differently, it is becoming more common for the adjective form to be used adverbially).

• Q2: 'analytical' is more common than 'analytic' in standard English (both meaning roughly the same thing 'able to think clearly and precisely'), but in math 'analytic' is a technical adjective form with a very precise meaning. The only instance I know of for 'analytical' is in the phrase 'analytic continuation'. There may well be other instances, but 'analytical continuation' is all I know of.