Why is 暗い conjugated as 暗くsometimes?


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!




### 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.

* * *

### 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

* * *

### 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)




### 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.

* * *

### 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

* * *

### 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)

