Thanks for asking a good question, Miss Xuy.

Whether a two syllable word takes “more” or “-er” is mainly a question of how the pattern of vowels and consonants would effect the rhythm of the word if the “-er” suffix were used, and it’s more of an art than a science. There are many words where both versions are used: common, handsome, stupid, healthy, etc. Native speakers sense this instinctually. Basically, if you can still pronounce the word as just two syllables after adding the suffix, it’s OK to do so. On the other hand, there are some one syllable words that don’t usually take the suffix, for example, participles. In American English, we often avoid using the -er even on one syllable words, if the word already ends in an R. I don’t know if this is true in non-rhotic accents, but pronouncing two “-er” syllables in a row sounds very awkward in US English. For this reason, I would say “more clear” in the spoken language, even though I would always write “clearer.” For this reason alone, I would avoid using “cleverer” in the spoken language. That being said, it would absolutely not surprise me to hear it from a native speaker, because the truth is that there is a lot of room for variation when it comes to two syllable words. In short, your nephew is doing fine, so congratulations.

I agree with Phil who has nailed the issue. As a non-rhotic speaker, I always say and writer "clearer", and it never occurred to me that Americans might say "more clear". But imagining the situation now, I can see why - I sound like a buffoon saying it to myself in the American way.

I would probably say "cleverer" but write "more clever".

I would never say or write "distanter" and I've never seen this anywhere. Perhaps there is a pattern relating to words ending in -ent or -ant.

I would always say "simpler" as this also only has two syllables, and I would say that this is the norm.

I think I would say and write "more common", but "commoner" sounds OK to me too.

Hi. Its a very tricky question indeed. You see, there is a simple way to get the comparative or superlative form of any adjective. Its the syllable. If the word is mono-syllabic, there should be an -er with the adjective to form the comparative form. Such as: Black- BBlacker. But when the world is poly-syllabic, there should be a -more used along with the adjective. Such as: Beautiful - More Beautiful. Coming to the word, Cleaver is clearly a poly-syllabic word. Clea+ver. So it should be like More Cleaver. But the interesting thing is, there are exceptions. There are few words which has both the ways to make comparative such as: cleaver cleverer. Narrow Narrower etc. 

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