A "participle" is a verb form that works like an adjective (or an adverb), such as "running" in "running water". A compound adjective is an adjective that is composed of several words, such as "brick-red" in "the brick-red door". An adjective may be both, as in "fire-breathing monster".

B should be "as many as 200 nation states", because it refers to a plural ("much" is used only for singular mass nouns). It means "up to 200", but probably pretty close. A means "201 or more".

In addition to the grammar suggestions above, note that there are a couple of punctuation problems: "This" needs to be capitalised, and you have a comma splice of two independent clauses which should be replaced by either a semicolon or a full stop. E.g. "This song is addictive; you have no idea of how many times I've listened to it."

Here are your options for verbs with "recommend" (and also "suggest", "demand" and other similar verbs): 1. Use a "that" clause (even if the word "that" is elided) + pronoun + subjunctive: "I recommend [that] you do...". (Note that you can't tell it's subjunctive, but you could for third person singular: "... that he do...".) 2. Use a "that" clause (even if the word "that" is elided) + pronoun + "should" + infinitive: "I recommend [that] you should do ...". 3. Use a gerund: "I recommend doing". You cannot use "to" + infinitive, with or without an associated pronoun. You'll find that 1 is more popular in the US and 2 is more popular in the UK, but they mean the same. The difference with 3 is that it does not say is who is doing the doing, unlike 1 and 2. It may be implied as "you" in your context, but you can imagine other contexts in which it might be "we": "I recommend declaring war on Ruritania".

Both are fine -- well done! A "hallmark", as you may have figured out, is a an official stamp on items in precious metal attesting to their origin and quality, but it is often used figuratively to mean "distinguishing feature" as you have used it. (Given that's its sense, you could omit the "one of the main", but that's just a stylistic thing.)

No, more likely not. As you have it, you are using "yet" as a conjunction, which often (but not always) joins two clauses together, and so winds up in the middle of the sentence. Your example would typically be written, " I like any type of music, yet I am waiting for someone with the same taste in music as me".

Well, if those are the choices, it should be "of" the society and "for" an achievement. But it sounds a bit repetitive to me. How about, simply, the "Korea Computer Graphics Society Grand Prize"?

In your example, you are looking for a conjunction that joins the clauses "tell the truth" and "I did" -- and note the capital I. You will come across some traditionalist grammarians who insist that it has to be "as" because they do not agree with "like" being used as a conjunction, only as a preposition, e.g. "Tell the truth, like me". But the truth of the matter is that this purported rule is not based on actual use of the language, and the conjunctive use of "like" is well established in English. So unless you are worried about those people's opinions, really either word can be used.