Why is 暗い conjugated as 暗くsometimes?

Question

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.

For instance:

もう暗くなったから、帰りましょう。

I'm a beginner so bear with me please and thanks!

Answer 1

い-adjectives

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.

you'd say

暗くない

If you want to say

It'll get dark.

you'd say

暗くなる

So, in the example sentence you provided

もう暗くなったから、帰りましょう。

Because it's already gotten dark, let's head home.

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な-adjectives

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.

you'd say

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

きれいになる

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Inflected forms for い-adjectives

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)