"This movie was not scary."
So you can say: えいが は こわくありません でした or えいが は こわくなかった です but not えいが は こわくなかった でした
In other words the adjective can be past tense or the verb, but not both? It seems odd that the adjective/verb tense doesn't match.
Can anyone explain why?
The adjective is what normally determines the tense (as well as positive or negative), with です merely adding politeness to the sentence.
こわい(です) = it is scary
こわくない(です) = it is not scary
こわかった(です) = it was scary
こわくなかった(です) = it was not scary.
However, when you take a more polite/formal denial, like こわくありません, it loses its ability to be conjugated into a past tense. This is when です takes over that function and becomes でした.
You should think of it as "ありませんでした" and "なかったです". ありません is the polite form of ない. The pairing of "かったです" is just a grammatical rule that should be memorized.
This one definitely threw me off... "この えいが は こわくない でした。" wouldn't be accepted here?
No, that mixes two different tenses. The i-adjective in itself kind of includes an auxiliary "to be", which can -and should- be conjugated. The adjective determines the (present/past) tense for the whole sentence. です is just an addition for politeness, and is only conjugated if it's the only verb in the sentence that can do so.
- こわい（です） ＝ it is scary
- こわくない （です）＝ it is not scary
こわくなかった（です）＝ it was not scary
こわくありません = it is not scary (more polite/formal)
- こわくありませんでした = it was not scary (idem)
I have seemed to found a bug. Even though I input the exact same sentence it is saying is the correct one, I still get it wrong
It may not really be a bug. Sometimes that happens to me when I have internet connection problems, and it stops once that gets fixed and I refresh the page.