To make a comparative form, one can add –er (as in ‘nearer’) in some cases or one can use the word “more” as a prefix (as in "more beautiful") in some others.
Is there any rule that says where can one add –er and where can one use “more” as a prefix?
This is not exactly an answer. This is an explanation of how one native speaker thinks about this.
Hopefully this will stimulate someone to post a better answer, maybe disproving what I say below.
I do not think about -er vs. more in terms of a rule regarding the number of syllables. Some people say that one-syllable words get -er and multi-syllable words get more, except all words ending in -y get -ier. That's probably right most of the time, but it sounds to me like a pseudo-rule similar to "i before e except after c", another rule that's often right but goes against the spirit of the language. English is more origin-, precedent-, and conflict-oriented than rule-oriented.
It didn't me long to think of funner, which is not a standard English word. Sometimes people say it, but even the first time I heard it (as a teenager), I cringed. To my native ear, it sounds ridiculous, suggesting that the person saying it just doesn't "get" English. Even people who say it know it sounds silly. Unfortunately, I don't know how I know that.
I think that when you are learning a foreign language, you should mainly be interested in how to perceive it the way a native does. So, for this reason, I think you shouldn't think about this in terms of a rule that goes by the number of syllables, even if it's approximately correct. Somehow, through exposure to many examples in a lot of real usage, natives figure out which words don't sound right with -er. Learning it that way will take a while, but what you learn will be real—the spirit of the language, not a "rule".
Here are some exceptions to keep in mind as you're considering hypotheses:
One-syllable adjectives that don't take -er:
fun, buff, lost, burnt
There may be a legitimate rule that past participles can't take -er.
Far sort of takes -er, but it becomes farther or further. Bad doesn't become badder (except in lower-class slang); it becomes worse. An interesting and unusual one is aft.
One hypothesis is that if the adjective is also a noun that you can "have", as in "have some fun", then it can't take -er, that's refuted by calm.
Multi-syllable adjectives that don't end in -y but can still take -er:
stupid, vapid, evil
Another observation: most adjectives that can take -er can also take more, but not all. For example more fast sounds weird--though you must say it in a weird comparison like "more fast than inquisitive".
A hypothesis that comes to mind is that Anglo-Saxon adjectives naturally make comparatives with -er and Latinate or loan words don't. Anglo-Saxon words usually have a different "feel" than imported words, which might affect how we alter their forms even if we don't consciously know their etymologies. Adjectives from Latin usually have two or more syllables (like "terrific", "circuitous", "accidental"). And Anglo-Saxon adjectives are often one-syllable (like "red", "clean", high") or end in -y. Unfortunately, this hypothesis runs into the ground with fun.
You might think this hypothesis is, er, stupider than the number-of-syllables hypothesis, but it might actually be in the vicinity of a genuine explanation for why some words feel natural with -er and others can only take more. This would be an explanation, not a rule. An explanation can shed light on cases you haven't encountered yet without pre-deciding them; other factors may enter into other situations.
Sure enough, this ngram shows that stupider got started long after more stupid. Apparently, the need to compare levels of stupidity was so great that people granted stupid a sort of honorary Anglo-Saxon status in order to use the more-convenient comparative -er.
And once stupider is in, by analogy vapider eventually starts sounding more acceptable. Vapider doesn't show up on Google ngrams, but dictionaries accept it.
This is what I mean by English being more origin-, precedent-, and conflict-oriented than rule-oriented. Word with different origins follow different rules, conflicts result, they get resolved, the resolutions set new precedents that people apply to other words, and on it goes.
Regarding other factors that affect some other words, the word fun might not take -er because even though fun is an adjective, we usually use it as a mass noun when speaking of degrees: "that was a lot of fun", "we could have a bit of fun". This might explain the feeling of unease at giving it -er. If the noun is felt to be primary, that may conflict with -er; but more works with mass nouns as well as adjectives.
Evidence against this hypothesis would be two-syllable Anglo-Saxon adjectives which don't take -er.
Neither the word evil nor the word vapid can take ER to make a comparative. The word stupid technically can take ER and EST but I don't know that I've ever heard a native speaker use the word stupider. I have definitely heard stupidest but not the ER. When I say the word evilest it doesn't sound too bad but once again the ER version sounds completely wrong to my native ear. Plus neither the comparative nor the superlative form of evil or vapid is in the dictionary.
The rule, if you can call it that, is for the most part single-syllable adjectives will take an "-er" in the comparative form (going to focus on comparative for the moment because the rules generally apply to the superlative as well, except in cases like stupid and fun).
Anything three syllables or longer will always take "more/most" etc, and will never add on an "-er/est".
As for the two-syllable adjectives, as was pointed out earlier, this is where it gets tricky.
What I've been able to deduce is this: most of your two-syllable adjectives will fall into the category of adding "-er" to them. There are, of course, some exceptions. But from what I can tell, the pattern is this: if you have the stress on the first part of the syllable, in general you are going to add "-er".
Two-syllable adjectives with the stress on the second syllable are very rare indeed (direct is an example, and as such you can see it does not get an "-er"...No one would ever say "He is directer than her", even if the spellcheck isn't marking it as incorrect; perhaps grammatically it works but in use it would never happen). There are, however, a list of words that end in two sounds that never get an "-er", and they are the "ick" sounds and the "ecks" sounds.
Complex, convex, basic, specific, hectic...These are all two-or-more-syllable adjectives and they all end in those sounds, and none of them will ever take an "-er" on the end. They all must use the "more/most" modifiers.
I do not know (and do not expect) this rule applies universally to the one-syllable adjectives (see quick-quicker but lax....laxer? Again, spellcheck doesn't mark it wrong but I am unsure).|
Thoughts?
The basic grammar rule is adjectives with one syllable have the comparison forms with -er and -est. Adjectives with three and more syllables take more and most. For adjectives with two syllables it's a bit more complicated, some take -er/-est, some take more/most. For details see a grammar. In some cases both forms of comparison are possible.
