was not
Structure
- Noun or な-Adjective +
- じゃなかった・ではなかった
- じゃありませんでした・ではありませんでした
When should じゃなかった be used and when should it just be なかった?
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Hey!
Sorry for the extremely slow answer 
なかった (with adequate conjugation) is used with adjectives, nouns and verbs.
じゃなかった is used after なadjectives and nouns.
くなかった is used with いadjectives
ない stem + なかった is used with verbs.
Examples:
- For verbs:
V(る5) → 座 る → 座ら なかった
V(う) → 歌 う → 歌わ なかった
V(く) → 歩 く → 歩か なかった
V(す) → 話 す → 話さ なかった
V(つ) → 打 つ → 打た なかった
V(ぬ) → 死 ぬ → 死な なかった
V(ぶ) → 飛 ぶ → 飛ば なかった
V(む) → 休 む → 休ま なかった
V(ぐ) → 泳 ぐ → 泳が なかった
Irregular Verbs
する→ しなかった
くる →こなかった - For いadjectives:
Adjective(い）- い +くなかった
かわいい → かわいくなかった - For なadjectives:
なadjective + じゃなかった
嫌いじゃなかった (嫌い is surprisingly なadjective!) - For noun:
Noun + じゃなかった
犬じゃなかった
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Is 暇 a noun or a na-adjective? I can only recall seeing it in constructions with +ある, so I thought the former, but in this example:
先週末、私は暇じゃなかった。
it’s taking the copula as if it were an adjective?
@ulurujamman Hey! 暇 is both a noun and a な-adjective depending on what you would like to express. It is similar to the English “I am free” and “I have free time.” You can say “I didn’t have free time.” or you could say “I wasn’t free.” Both がなかった and じゃなかった can work in this example, but the nuance changes slightly. Cheers!
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I struggle with です and あります.
I keep answering “赤色 ではありませんでした” (and similar questions) wrong, thinking it should 赤色でした. I feel like it’s so similar to say (conveniently, from the same review session) 教室は静か でした. Is there any good online resource that could help me out? It’s this particular construction I’m struggling with. これは赤です I’d probably get right (and if it’s say, “There is a cow” type sentence, that I would hopefully get right).
Another example is 宿題は大変たいへん ではありませんでした.
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@pasi
Hey and welcome on the community forums! 
I struggle with です and あります.
Actally, it’s a struggle with だ’s past forms. I will try to help you understand them better 
But you will stop confusing them naturally with more experience, so don’t worry!
Let’s start with basic affirmative conjugations:
| Short form | Polite form | Translation |
|---|---|---|
| だ | です | to be (is/are) |
| だった | でした | was/were |
As you can see, です is a polite form of だ.
The past forms of だ is だった. でした is both past AND polite.
本だ。 It is a book. 本です has the same meaning, but with added politeness.
本だった。It was a book. 本でした has the same meaning. But with added politeness.
Ok, time for negative conjugations:
| Short form | Polite form | Translation |
|---|---|---|
| ではない・じゃない | ではありません・じゃありません・ではないです・じゃないです | is not/are not |
| ではなかった・じゃなかった | ではありませんでした・じゃありませんでした・ではなかったです・じゃなかったです | wasn’t/weren’t |
I will try to explain some background so it would be easier for you.
First of all, notice how similar the negative conjugation of だ is to the negative conjugation of verb ある (to exist/to have).
| Short form | Polite form | Translation |
|---|---|---|
| ある | あります | to exist/to have |
| あった | ありました | existed/had |
| Short form | Polite form | Translation |
|---|---|---|
| ない | ありません・ないです | not exist/not have |
| なかった | ありませんでした・なかったです | did not exist/did not have |
However, while those are similar negative forms are identical, だ and ある are different words.
Ok, then the question is, how the heck the negative form of something as simple でした (meaning “was”/“were”) is something like ではありませんでした？ It is not even close. And why is it so similar to the negative form of ある?
The answer lies in the である which is the original form of which “だ” is a contraction. It is nowadays mostly used in formal writing and speeches. As you can see, it is basically identical to ある, and it explains why the negative conjugations are so similar. Basically, the affirmative form has changed a bit, but negative remains. By the way, である itself is also contraction, but it is another story.
| Short form | Polite form | Translation |
|---|---|---|
| である | であります | to be |
| であった | でありました | was/were |
| Short form | Polite form | Translation |
|---|---|---|
| ではない | ではありません | is not/are not |
| ではなかった | ではありませんでした・ではなかったです | was not/were not |
Ok, but why there is は in negative form? Why it’s not でない?
And what the hell is じゃ？
Well, particle は, we know as topic particle also has a contrasting function. It is most visible with が.
ロイさんはペンを持っていますが、リザさんは持っていません。
The contrasted elements (Roy and Riza) are marked with は. That is the basic contrasting function.
So adding は to でない adds contrast, basically contrast to something that is not the case. It was added so often in a past that it became “one” word, and I think it mostly lost that contrast nuance, it just became a negative form.
じゃ is simply a casual form of では. Some older people simply pronounce だ as じゃ. By the way, contraction of ては is ちゃ.
Ok, so why the negative past form has two polite forms ありません and でした？ It is so easy to confuse with でした, the past form!
Well, originally, there were many competing negative past forms, and this is the one that became the major one. You just have to remember it. But the mroe you will see of it, the easier it will become. Just know that if there is ありません before でした then its negative past form
I hope it helps,
Cheers
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Wow. That is a mighty detailed reply. Thank you so much. I won’t even pretend I read and understood it all yet but I shall go through it in detail.
Also thanks for the superb service. 
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By the way, I have made a small error in translation, it should be like below (already fixed in post above):
So である has the same meaning as だ while ある means “to exist/to have”
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Hey, just wanted to let you know that your answer was actually very, very helpful. I haven’t actually got a single related grammar item wrong in my reviews since (I’m sure I will but it’s a huge improvement). Thank you so much.
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Hi! I’m confused on when to use this " じゃありませんでした" and when to use ”じゃない". In the sentence below why is " じゃありませんでした" the correct usage? Thank you in advance
朝あさごはんを作つくったのは、お父とうさん じゃありませんでした
@lorenzosama Hey! It really comes down to what level of politeness you want to use/wish to express. This particular review question actually accepts all of the following answers:
じゃなかった
ではなかった
じゃなかったです
ではなかったです
じゃありませんでした
ではありませんでした
When it comes to choosing the best answer, it would be best to refer to the post made by @mrnoone above. Let us know if you have any further questions. Cheers!
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The accepted answer for the following question is shown
風邪を [ひくつもりはなかった]
Why is じゃなかった not accepted for the past tense as じゃない is shown in the grammar notes for the present tense?