Adjectives and tenses

Japanese Language Grammar
1

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My answer is the top one, and the correct, bottom. Honestly I don’t know how it works. I thought it was more interchangeable but now I’m not sure. Is the rule that the adjective’s tense changes rather than desu?

2

Imagine that ない is a い-adjective, because that’s how it behaves during conjugation.

So in the past tense you have:
安かった (past tense of 安い)
安くなかった (past tense of 安くない)

It’s also worth mentioning that じゃなかったです is kind of the least polite of the possible polite conjugations. The more polite forms are:

  • ではありませんでした (most polite)
  • じゃありませんでした (slightly less polite)

Also, yes. です is added for politeness in most cases and only its past tense でした is mandatory if the preceding word doesn’t define a tense.

3

I guess the rule of thumb is that you use non-desu tools if you can to express past tense (like katta), and if that’s not possible, you go for deshita. But don’t quote me on that. The course I took was very… non-scholastic you could call it I guess, even too much at times.