I think what SaulM means is that adding "er" and "more" are both used in the English language. Some words, "young", "smart", etc. require the "er" suffix. Other adjectives only use "more", like "more interesting" or "more intelligent". I can't think of any cases where either or could be used.
One syllable words get the "-er" suffix. Words with three or more syllables get "more" prefixed. Two syllable words are one or the other. I don't think there are any words where you can choose which form to use.
Shakespeare and the King James Bible, generally considered the touchstone of modern English, used double comparatives: "This is the most unkindest cut of all" (Julius Caesar), "more nearer" (Hamlet), most straitest (Acts 26:5) &c. The disuse is because of grammarians, like Lyndley Murray and their lower middle class neophytes who didn't wish to sound common. One often notices the petit bourgeois hyper-correction using "I" as an object.
Be proud of the language in all its dialects and form, the brilliance of Burton (The Anatomy of Melancholy) and Burton (Where eagles dare).
A detailed lesson would be nice, but I'm pretty content with learning through trial and error and through peer comments. I never really absorb actual lessons unless I keep failing until I learn the rules. It's like being thrown into a card game that you've never played before. You pick up the rules as you play it.
So I just dropped you off in the middle of latin america. You now will immerse yourself in the language or you will go without what you need. This is an online virtual immersion program. No guides, no dictionaries just tips here and there and your fellow students. This is a good thing and a new way to learn. That's why DL has excellent reputation in doing translations of websites. I hope you embrace it.
Sounds like you're asking for a guide to English! What is called the comparative for obvious reasons, used to compare TWO things, is made by putting -er on end of a short adjective - thus: bigger, taller, older, younger - maybe with a tiny modification if adjective ends in -Y, so uglier, happier. And put "more" in front, cf más, for longer adjectives - I'd say anything with 3 or more syllables definitely use "more" in fact, off top of my head, even 2 syllables if it it doesn't end in -y eg more perfect.
As a native speaker (English) I would find "more young" grammatically awkward! Looking around on this I found a rule which makes sense to me: if the adjective is 1 syllable then the comparative should be adjective+er: I.e. young - younger, old - older, but if the adjective is 2 or more syllables then the comparative should use "more + adjective" e.g. respectable - more respectable, beautiful - more beautiful.
This is usually true, but watch for exceptions. This is English, remember. Everything has exceptions. You can't add -er to 'fun', for instance.
EDIT: I have another counterexample that works the other way. 'Happy', which has two syllables, does take the comparative form 'happier'.
EDIT 2: I think the guideline should just be rephrased to say and 1- or 2-syllable adjectives can usually take the -er comparative.
Actually, not quite. The comma in "younger, naturally" is important. it implies that this might be the answer to someone else's question or comment. Q: "Are you older or younger than your sister?" A: "Younger, naturally" In this answer there is a slight implication that the question should be unnecessary because obviously I look younger. "younger naturally" without the comma sounds like it might be a phrase out of a makeup advertisement --- "Use our makeup and you will look younger naturally" (as if no one will notice the makeup). However, context is everything. Hope this helps.