I think the rule is that if the adjective has more than two syllables, you aren't supposed to use the endings -er/est etc. Also, you shouldn't use the endings along with the words (more/most etc) in front (such as more cleverer), and words with 1 syllable usually are special cases, such as good/better/best.
Clever having two syllables is ok; it only looks weird because of the -erer (but it sounds fine to me, adding -est also sounds natural (which is probably a good test) cleverest).
It's when you get into 3 syllable ones like prestigious that you can't use prestigiouser, and have to use more prestigious.
That's the rule I learnt at school: One syllable, you always add -er (except in irregular comparatives). Two syllabes, you have the choice between -er and "more". Three, you need to use "more". I guess it all depends on how good my teachers' English was.
- Gradable adjectives can be used in their comparative and superlative forms, whether using a periphrasis (more, the most) or an inflection (-er, -est).
The inflection with disyllabic adjectives is used when the last syllable is unstressed (therefore: happy - happier, easy - easier), when they end with a syllabic "l" (simple - simpler) or with an "r" (clever-cleverer).
Trisyllabic or longer adjectives take the periphrastic form unless they are negations of disyllabic adjectives with the prefix un- (unhappy - unhappier).
This is all theoretical (what Quirk says about comparatives and superlatives). The practice might be different, as you can see in the US and Canada they prefer the periphrastic forms.
In her first post Yolanda stated the English rule very concisely. But some people obviously feel a greater discomfort with appending "er" to a liquid consonant (English "r", which is represented phonetically as an upside-dow lower case r] and "l".) The same awkwardness is encountered with "leveler" versus "more level".
As she indicated, this problem does not come into play with the superlative ("est").
There are partisans for each. I myself prefer cleverer, but would not say that "more clever" is incorrect, although to me it sounds a little weird.
I found this on the web (www.[B]oed.com[/B]/learning/a-level/lessons.html )
"Worksheets for A level English Language - Exercises for age 16-18 ...
The fact that the OED is a historical dictionary means that once a word is included in it, .... What makes one example cleverer than another? ..."
. . . and also this:
Merriam-Webster's Learner's Dictionary (www.learnersdictionary.com)
Inflected forms:clev·er·er; clev·er·est
Meaning:[also more clev*er; most clev*er] 1 : intelligent and able to learn things quickly
You can use "more clever" or "cleverer", as the comparative of clever; and you can use "most clever" or "cleverest," as the superlative of clever. However, it is safer to use "more" in most cases when dealing with anything other than a one syllable root word as I will explain below.
In English the decision on when to use -er/more, and -est/most is based on two things: the number of syllables in the root word, and the ending of the root word.
The general rule is, as other members have pointed out, if the root word is one syllable you use the suffix -er, if the root word is three syllables or longer you always use more and not the -er suffix, but in the case of two syllable words, like clever, you can use either depending on the ending of the root word.
Two syllable words which end in: -less, -ful, -ing, -ous, -ant, -ed are never modified with the -er suffix and must to use "more". For example, "useful" cannot be usefuler, something is "more useful"; or someone can be "more pleasant" but not "pleasanter"; or "more boring" not "boringer", and so on.
While the two rules above (syllables and root ending) are hard and fast rules, what I will explain below is a matter of taste, preference, or practice.
In the case of clever, its ending of -er, does not require the use of more, but there was a period of time, at least it the U.S., where the -er ending, the -le ending, and the -n ending were all included in the above, and for that reason "more clever" probably has a more natural sound than "cleverer" to most people. Examples of this for the other endings are "simpler/more simple", "more fun/funner". Since this was taught for some time, using the -er suffix fell out of practice for words with these endings, which means that while correct under either -er/more option, their unusual use seems to make people uneasy, as though it were something a child would say. It is not technically incorrect to say "cleverer", just odd sounding to many people.
Also, since another use of -er in English, is to convert a verb into a noun of someone performing the verb action, for example the verb: to speak can become the noun: the speaker simply by adding -er, it is helpful to avoid applying -er when the word could be confused as this type of -er noun, rather than an -er comparative adjective. For example, even though the adjective common (two syllables without a special root ending) can be modified as "commoner," you would want to make sure that your context is clear so that "commoner" does not sound like you are referring to the noun meaning an ordinary person. When in doubt, or to be clear, you could just say "more common."
Furthermore, when a comparative is combined with another modifying phase, and the root word is two syllables or longer, it is usually preferred to use "more" or "most', rather than "-er" or "-est". For example, if I wanted to emphasize just how clever someone was in relation to someone else by using "a lot" or "much" it is less common to use the "-er" suffix. Example: "Sally is a lot more clever than I originally thought." sounds better than "Sally is a lot cleverer than I originally thought". Or, "Dan is much more clever than Steve." is generally preferred over, "Dan is much cleverer than Steve." This does not apply to one syllable root words, however, which do use the -er suffix even when accompanied with an modifying phrase. If you want an example, replace strong/stronger into the above example; native speakers would prefer to use "stronger."
So my long explanation boils down to this: you can either either, but since there are so many circumstances, due to endings, or modifying phrases, that can cause a two-syllable word to require "more" instead of "-er", I generally default to using "more" which is a perfectly correct option for anything but a one syllable root word adjective.