Showing reason is a very complex matter in Japanese, but these three items, two of which are related to each other, will give you a basic understanding of how to do this.
のだ decisively shows reason. It may strongly emphasize an exclamation, reasoning, cause, grounds, or desire. It is used when, for instance, you are responding to having been asked for a reason or explanation. のだ allows the speaker to not become detached in conversation. However, it can't be used to just state a fact. It's going to show reasoning of some sort.
You can see it as のだ (more literary), んだ (spoken), のである (literary), and のでございます (honorific form). It must follow the 連体形 of an adjective or verb. Therefore, with nouns you'll use なのだ, which uses the copula.
のか・[のですか・んですか] is the question form, which inquires why and is perfect for asking for a reason or explanation. In certain situations it may not be appropriate because of tone. の alone can also make a question in casual speech. It shows reason with a falling intonation in feminine speech. As expected, のだ may be used to create a blunt question.
The following chart lists the following potential conjugations involving tense and or negation for ～のだ. The chart below shows these forms for when ～のだ is used with a nominal phrase. For anything else, simply take away any existing な. As will be discussed in more detail, this chart does not imply each form below is equally used and natural in any circumstance. It is important to not go and run with this but be cautious as to what is frequently used and when.
This chart is for the sake of having all of these basic forms represented. It does not imply that all of these phrases above could even work with any given noun such as 犬. All of these forms are grammatically correct, though some of them would need a lot of context to be natural, and the language for some of them may have to be more old-fashioned to work.
Patterns italicized are rarer and literary. If bold and italicized, it means that it is rare/older as shown, but it may be frequently used with ～ん instead of ～の. ～のだった is like a rather forceful and also explanatory "it so happened that".
～のじゃない, just like ～じゃない, is often used to ask for verification, but in this sense you are wanting explanation. Even so, you'd be hard pressed to hear ～のじゃない？ Instead, you would hear ～んじゃない? If you wanted the の, ～のではないか would be your next best option. However, this form is more indicative of 書き言葉 or very serious utterances. You should separate this from the のじゃない that comes about from using の as a dummy noun and then following it with the copula.
～のじゃなかった is like "it wasn't that". Again, in actually speaking this form as well is usually んじゃなかった. This is the case for all of these forms.
ん is 話し言葉的 and の is 書き言葉的. This statement is for this grammatical circumstance, but it is also true that ん is more colloquial in nature where ever it shows up in Japanese. This statement does not also deny the potential of の being used in these forms at all in the spoken language. As you will see, ～のです and ～のですか do get used in careful politeness speech, but ～んです and ～んですか are certainly more prevalent.
I didn't lose.
Do you not have a cheaper one?
You’re not going, right?
Aren't you going?
That water was not to drink.
Will you be coming next Saturday?
I caught a cold. (Reason)
I won't drink coffee now. I am going to bed soon.
Sorry, I missed my ride (vehicle).
When is the rainy season over?
"Why do you like natto?" "It's delicious."
My friend is waiting.
Particle Note: The particle ので is actually just the て form of this. It shows specific reasoning and is translated as "because/since."
The most important usage of the conjunctive particle から is to connect clauses to mean "because". It shows reason and may solicit a response, suggest something, or any means that appeals to the listener. If the sentence is polite, the clause it modifies should be too.
から doesn't have to be used after a dependent clause to show reason. It may also be at the end of a sentence. When at the end of the sentence and followed by だ, you can make a strong assertion. Without a copula, though, you show reason with more emotional appeal. Regardless, if から is used with nouns or 形容動詞, it must be preceded by the copula!
から goes after the 終止形. So, you should use it after verbs, adjectives, or a copula phrase.
In Standard Japanese, you cannot say なから. However, it does exist in certain dialects. For the purposes of Tokyo speech, it is deemed incorrect.
For some of these examples, a question would be stated before the response in order to sound perfectly natural.
Since we have time, how about taking a break?
It's because the machine broke.
It's because I have an exam tomorrow.
It's because it's easy.
17. あいつが馬鹿だからだな。 (Masculine)
It's because he's an idiot, you know.
から can mean "because" and のだ strongly shows reason. Thus, んだから has it that the listener knows the "why". The speaker, then, reasserts. If the listener doesn't know the "why", then it's extremely pushy. A clause following のだから often expresses the speaker's judgment, intent, wish, etc.
18. やべーから、早くしろ！時間(が)ねーんだよ。すでに遅れてんだから、早よ準備しろ！(Casual; vulgar)
We're screwed, so hurry up! We don't have time! We're already late, so get ready already!
～たら makes a hypothetical. ねーんだよ means the same thing as ないんだよ. It is just more vulgar. ～てんだ is a contraction of ～ているんだ.
ので, んで in casual speech, is used to show reason or cause. Unlike から, it must only be used in this way. So, you should not use it with an imperative of any sort. Also unlike から, ので is used to show reason of present circumstance where there is no control in the matter on your part. It is objective in nature, and so it's also weaker. Appropriate translations include "since", and "because".
Unlike から, ので follows the 連体形 of adjectives, verbs, and copula phrases.
*1: ですので is technically incorrect Japanese, but it is has become frequently used to the point that is now correct Japanese.
*2: Although improper in Standard Japanese, だので is actually seen, but it's dialectical.
This rule, though, is relatively new in Japanese, and it really has no bearing over ので. In fact, you can also find ～ますので. This, however, will be considered ungrammatical by more speakers than ですので. Why this is the case is not clear, but it probably has to deal with part of speech and the formal nature of ので. If someone doesn't like ～ますので, the person is probably from West Japan.
Since it is warm today, the cherry blossoms will likely be in full bloom.
Since I finished my homework, I went to play outside.
Since that dish wasn't as a good as I thought it would be, I didn't eat more than a bite.
Because he said he wanted to go to an art museum, I took him around.
Since it was so cold, I turned on the heater.
As I want to become good at Japanese, I'll study with all my might.
“How much will it cost (to send) for the package?” “Since it is 600 grams, it will be 1500 yen”.
When at the end of a sentence, ので is essentially a softer version of のだ.25. すまない、もう先約があるんで。
I need to catch the last train.
Sentence Note: Ex. 26 is in response to a question.