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I'm having trouble understanding the best way to respond to someone asking
"why".

My study guide advised,

> To answer the question with "why", make a statement sentence that indicates
> the reason and add からです at the end,  
>  A: どうして昼ご飯を食べなかったんですか B: ダイエットをしているからです

Feeling like it couldn't possibly be that simple, I went looking to see if
anyone had explained things in more detail.

However, nearly every sentence with から I found are sentences that describe
both the cause and response, seemingly not as a response to a "why" question,
such as [this sentence taken from another
post](https://japanese.stackexchange.com/q/4022/6840)

> あのラーメン屋は人を待たせるから、行かないことにした。

I haven't been able to find much at all on からです in direct response to a
question, as it was in my book's example.

I figured the most likely reason for this was either  
A. からです is a nonstandard approach and my book is weird, or  
B. I've missed something.

Unfortunately, B prompted me to wonder if some of these descriptions actually
were a direct response that I was misunderstanding.

So, while I was searching for something like

> A： どうして外に行かなかったんですか  
>  B： 雨が降っていたからです

People were actually saying

> A： どうして外に行かなかったんですか  
>  B： 雨が降っていたから、外に行きませんでした

* * *

In short, is something like "ダイエットをしているからです" actually a standard way to give a
reason in response to a question? If not, what would be more appropriate?

Any other ways to give a reason are welcome, because nuance is neat.
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2012-08-17T01:17:05.130
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question
usage nuances
What's the standard way of explaining reason in response to the question "why?"? Do you use からです at the end?
Look up these expression: ～ものですから （formal) ～もんだ (casual)

<http://www.guidetojapanese.org/learn/grammar/genericnouns> (i.e. もの)
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2012-08-17T13:05:55.670
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answer
I think we'd say/write like this when we need to sound formal/polite:

>   * ダイエット（を）して（い）るからです/雨が降って（い）たからです
>

We normally say 「ダイエットしてるんです。」「雨が降ってたんです。」etc. in polite speech

And in casual speech:

>   * 「ダイエットしてるの。」(feminine)  
>
>   * 「ダイエットしてるんだ。」(masculine)  
>
>   * 「ダイエットしてるんだもん。」(feminine, cute)  
>
>   * 「ダイエットしてるから。」  
>
>   * 「ダイエットしてるもんで。」  
>
>   * 「雨が降ってたの。」(feminine)  
>
>   * 「雨が降ってたんだ。」(masculine)  
>
>   * 「雨が降ってたんだもん。」(feminine, cute)  
>
>   * 「雨が降ってたから。」  
>
>   * 「雨が降ってたもんで。」et cetera
>

The が after 雨 can often be left out when we talk casually.  

~~てるんだ/です in ダイエットしてるんだ/です and ~~てたんだ/です in 降ってたんだ/です are shortened
(contracted?) forms for ~~ているのだ/です and ~~ていたのだ/です, respectively. (The "い" and
the "o" in "の" are dropped.)
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2012-08-17T13:54:16.977
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answer