"That party was not much fun."
I feel like they are treating 'fun' as an entity which makes it more significant that it does not exist! Similar to how we would say "No one was at the party", 'fun' may be treated here as something that is found at the party and the ありません is intended as emphasis on the lack of 'fun'.
Just a theory, but someone please chime in if you know for sure.
while a good theory, it's just not the case. What they are doing here is taking apart たのしくない into たのしく and ない, which is incorrect because たのしくない is all one piece. It's the negated present conjugation of たのしい. With Adjectives like たのしい, you conjugate them and not です. You don't need to change くない that is the accepted negated present tense conjugation for い adjectives. To formalize it you just stick on the です and call it a day.
This course usually uses 丁寧語, the formal versions. But 楽しくなかった or 楽しくなかったです are totally ok too!
ありません is the formal form of ない The past tense of ません is ませんでした The past tense of ない is なかった Therefore, 楽しくありませんでした and 楽しくなかった are equivalent in meaning, just different in formality
I think its because you slapped the です on at the end and its not the right tense. For past tense it would be でした but putting that in would be redundant because the なかった takes its place and makes the sentence past tense.
No, かった Is the past tense and the correct form would be to add です At the end of that.
The statements convey basically the same idea, but your sentence would be more correctly translated as そのパーティーではあまりたのしくなかったです。