です is crystal clear but what is not clear are the occasions when we use んです and　なんです。　As far as I know the んです　and なんです are the same thing.
As far as I know they are used to >give reason< when person asks something but this does not seem to be its only usage thus the confusion.

そうです is clear, What is not clear enough are the different usese of そうですね and そうなんですね.
One meaning is "yeah isn't it", but are there others too? Just how do I interpret the ん　and なん　before です?

Than comes でしょう。
Which has keeps me most confused. Why can we not use かもしれない or もしかしたら　where we use でしょう? Since でしょう　is used to express possibility, that something may/might happen right?

Than comes the other confusion which is そうです　vs そうでしょう, そうですね　vs そうでしょうね, そうですか vs そうでしょうか。

Wow, that is a lot of です。 I will try to give you my understanding of these, so if anyone else wants to correct me, you are welcome to. These are all really versatile words, so there are tons of meanings and situations.

Let's start with んです... You have two main usages. One is as you said, to give reason. In this context something like 「食べたんですか」 Could mean "Is it that you want to eat?"
The other usage is just making things more polite. 行きたいです (I want to go) vs　行きたんです (It's that I want to go). They say the same thing, but the んです。 makes it more polite.
As another quick note んです is actually an abbreviation of のです which is why you have なんです it is used everywhere that you might use な instead of の. There is also the gerund form ので with an identical meaning.

そう

Adding ね to the end of そうですね makes the sentence seek or express agreement. Kind of like saying "that's right isn't it?" You wouldn't use as a question though, the meaning is more that you assume the person agrees with you.
Example:
いい天気ですね？ (Nice weather isn't it?)
そうですね。 (Yeah it is.)

そうなんです Just changes the nuance a bit. You might use this for saying something like "So that's how it is" or "So that's why...."
事故で電車だから田中が遅くになりました。 (There was a train accident, so Tanaka will be late)
あ、そうなんです。 (Oh, so that's why he isn't here yet.)

でしょう

The main difference between でしょう and かも知らない is that でしょう expresses wondering about something, where かも知らない expresses not knowing. I feel like using かも知りません kind of removes blame to yourself, making the possibility more like hearsay or uncertainty than your opinion.
美味しいでしょう (It seems tasty)
美味しいかも知りません (It might be tasty, but I don't know...)

Your last confusion with all the でしょう can get confusing if you think too hard. でしょう can be used to soften a sentence, making is more polite (the more you express uncertainty, the more polite you become)
So in そうです　(it's like that) and そうでしょう (That's probably how it is,) the latter is less direct, and more polite. A lot of these can be really different depending on context, I think with practice, you just start to get a feeling of which is which and what is proper.
Also remember lots of people (me at one time, included) Overuse ね at the end of sentences. It would be the equivalent of someone in English who says "like" a lot.

Don't sweat most of these. They seem really confusing when you are looking for exact meanings, but they are so common, you will start to understand them naturally with time.

syntoad Wrote:As another quick note んです is actually an abbreviation of のです which is why you have なんです it is used everywhere that you might use な instead of の.

Nearly. なんです is of course short for な　の　です

So you have なんです anywhere that you might have なのです (the former is more common, but the latter may be easier to look up in grammar guides and textbooks).

You cannot have なです because な is a form of the copula as well (along with だ and で ... です isn't really a form of 'the' copula but is another copula-like word. So rules for だ・な・で are very similar to each other.)

And, of course, sometimes なんです is 何です, but those cases should be obvious.

Also, of course, watch for what's before the そう, sometimes it's not the word そう but the suffix そう like this example snagged from ALC (the first suffix そうなんです I found):

Quote:うちの息子、最近寂しそうなんですよ…ほら、学校に行き始めたじゃないですか。それで他の子どもには兄弟や姉妹がいるって分かったらしいんです。
My son seems lonely these days... You know, he started going to school, and he realized （that） other kids have brothers and sisters.

There's a lot more, but, honestly... this is asking basically 'how does half of the Japanese language work' ... the copula(s) in their various forms and the nominalizing/explanatory の is an enormous chunk of Japanese grammar. You just have to break it down into one usage at a time, digest one and move to the next. (Or sometimes, two pieces at the same time when the really need to be compared closely.)

Thank you SOOOooo much. I have much better Idea about そう. But all these variants of です and でしょう were making me too much confused. Too much I must say.

Spammer -.-

I keep the whole thing around です easy, by simply remembering that anything with ん・の・なん means something along the lines of explaining/reasoning, as syntoad already wrote. :)