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I have seen this expression a few times, and have never quite understood it. In particular, it seems to be equatable both to ですから and to んです, but I can't figure out where it would be used over either of those two expressions. Here is an example I found on Google:

新聞に目を通していないと、仕事が 成立しないからです。

What does the からです mean in this sentence? And more importantly, when is it appropriate to use, and how does it differ from ですから and んです?

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2 Answers

up vote 5 down vote accepted

And more importantly, when is it appropriate to use,

Almost always, although it may sound a little bit informal. When asked "なぜ" or "どうして", you'd answer with からです。

なぜ遅れていますか?

なぜなら、目覚まし時計が壊れていたからです。

(Why are you late? Because my alarm clock is broken)

and how does it differ from ですから and んです?

ですから is a connective, not a final part. "Aですから、B". If there's no B coming afterwards, thenn there's a problem. It's a bit like "since" in English. You don't just say "Since I like ice cream," but you say "(B) I bought an ice cream (A) since I like ice cream."

んです is "のです" used more colloquially. It's like からです, and (in "のです" form) more formal. Also more common, as far as I can tell.

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Oops. I've always used ですから in lieu of からです, then. I guess people understood and didn't really bother to correct me. –  Ataraxia Sep 24 '12 at 3:23
    
んです is "のです" used more colloquially. Yes. This is all about it. –  Teno Sep 24 '12 at 5:13
    
It is not true that a sentence cannot end with ですから. See my answer for an explanation of why this happens, with an example. –  Tsuyoshi Ito Sep 24 '12 at 9:29
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@TsuyoshiIto: I can't help thinking it's not really a ending. To me, it's like saying "Because it is beautiful, you should see it (とても美しいですから、見るといいですよ)", but in a more oral fashion, where the justification comes after by pure effect of style. It's not "invalid" per say, it's just saying "B. Aですから" instead of 'normal' "Aですから、B". –  Axioplase Sep 25 '12 at 1:44
    
I think that you are right, or at least I share the same feeling after reading your comment. The construct like 見るといいですよ。とても美しいですから。 should be considered as anastrophe; see also this question. –  Tsuyoshi Ito Sep 25 '12 at 2:16
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Your translation is not correct. It seems that there are a few misunderstandings involved here.

First, “AとB” here means “B if A.” 仕事が成立しない means “I cannot do my job.”

新聞に目を通していないと、仕事が成立しない。 I cannot do my job without skimming the newspaper.

Next, ~から means “because ….” Therefore, your sentence is

新聞に目を通していないと、仕事が成立しないからです。 It is because I cannot do my job without skimming the newspaper.

Note that the whole sentence has the form Xです, where X = 新聞に目を通していないと、仕事が成立しないから. The subject of Xです is not stated, but for example the whole sentence can be a reply to a question “Why do you subscribe to a newspaper?”

As you can see, ~からです is used to state a reason for something.

お菓子を食べたのは、おなかがすいていたからです。 The reason I ate snacks is because I was hungry.

A: どうしてお菓子を食べたのですか。 Why did you eat snacks?
B: おなかがすいていたからです。 Because I was hungry.

In colloquial context, です is often omitted. In this case, we can use polite form before から.

A: どうしてお菓子を食べたのですか。 Why did you eat snacks?
B: おなかがすいていたから。 / おなかがすいていましたから。 Because I was hungry.

If the clause before から is a sentence ending with an adjective or a copula, its polite form ends with です, and you will end up with ですから.

函館山からの夜景を見るといいですよ。とても美しいですから。 You should see the night view from Mt. Hakodate. (This is because) it is very beautiful.

んです does not state that it is a reason. Depending on context, it can sometimes replace からです, though.

A: どうしてお菓子を食べたのですか。 Why did you eat snacks?
B: おなかがすいていたんです。 (Because) I was hungry.

Here おなかがすいていたんです literally just means “I was hungry,” but from the context, it is clear that it is stated as a reason why B ate snacks.

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Removing the incorrect translation, as I think it might be a bit distracting. –  Ataraxia Sep 24 '12 at 0:50
    
@Tsuyoshi, When you say "we can use polite form before から", do you mean that ~ですからです (double polite) is valid? –  Pacerier Oct 31 '13 at 13:36
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