I know in past tense, you use く like 暗くない but even in present tense, I see example phrases where 暗く is used.
I'm a beginner so bear with me please and thanks!
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For い-adjectives the く-form is used in a number of contexts.
As you noted, if you wanted to say
It's not dark.
If you want to say
It'll get dark.
So, in the example sentence you provided
Because it's already gotten dark, let's head home.
And for completeness sake, let's look at how this would correspondingly work for な-adjectives, like きれい
If you wanted to say,
It's not pretty.
きれいではない (less formally) きれいじゃない
If you wanted to say
It'll become pretty
then you'd say
And because you mentioned the past tense, there are a few more points to be made.
Adjectives (like verbs) inflect in Japanese. For い-adjectives, the inflected form (if it's not plain non-past), the い is dropped and replaced with k. There are then three different usages that follow this pattern
Past tense: くらい => くらかった (it's dark)
Conditional form: くらい => くらければ (if it's dark)
and the く-form you've been encountering
to form a negative: くらくない (it's not dark)
to describe a changing state: くらくなる (it'll get dark)