Translation:This movie was not scary.
Okay, this is bugging me, I'm very confused... Why is it こわくありませんでした? Saying 'this movie is scary' would be このえいがはこわいです, right? Wouldn't 'this movie was scary' be このえいがはこわいでした? So, um, 'this movie was not scary' should be, logically, このえいがはこわくないでした? Or is the ありません specifically for the past-tense? Maybe this is just because I'm relatively new to this...
the past form of an い adjective does not rely on changing です into でした. です is simply a copula to add politeness and formality to the sentence. these adjectives have their own conjugation.
- non-past affirmative: ~い
- non-past negative: ~くない
- past affirmative: ~かった
- past negative: ~くなかった
from these plain forms, です is then attached to (or not), depending on the level of formality expressed. another way to conjugate い adjectives is to use ~ありません in negative forms.
- non-past affirmative: ~いです
- non-past negative: ~くありません
- past affirmative: ~かったです
- past negative: ~くありませんでした
only because ~ありません cannot take both negative and past forms, でした is needed to indicate the past tense.
They're just two ways of writing it formally/politely. You're right in saying it would be こわいです when something is scary.
Then when you have こわくない, 'ない' is actually the negative form of the verb ある added to the end. Now you can either change ない into ありません, or you can add です at the end to make it formal/polite.