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 ...
Doing some research, I've gathered the following:
When the predicate in an adverbial clause is stative in nature, its form does not change, regardless of the form in the main clause.
"I wear a coat when it's cold."
"I wore a coat when it was cold."
Japanese Stage-Step Course: Grammar ...
The following is my intuition.
The pattern is Aとき, B, where A is an adjective.
(1) If you mean Whenever A, B, then both A and B are in dictionary form.
暑い時はビールがおいしい Whenever it is hot, beer tastes good.
In this case 暑かったとき is not possible.
(2) If you mean an event in the past as When A, B happened, then B is in ta-form and A can be either in ta-form or in ...
AFAIK, there is no real difference between adjectives and verbs in terms of when to use their past-tense or non-past forms with 時. Both forms are valid and useful, but they do not mean the same thing.
I think the trick here is to understand that there's nothing really special (grammatically) about the 時 constructs: 時 is just a noun, which means "(a/...
Let's look at each of your sample sentences in turn.
Breaking this down word for word:
eating-thing → food
deliciousness (as a degree or amount)
[copula]: is, are
Putting this together:
Is food [a degree of] deliciousness?
That doesn't make a lot of sense, ...
Let's look at your sentences and explore their meaning.
This says, "Is the food tastiness?" I don't think that's what you want to say.
This could possibly work, but it still sounds a bit off to me. (But, I'm not a native speaker.) What strikes me odd here is the use of the particle が. Had you written it as
Adverbs describe the manner in which something happens. In your example sentence, there is no manner being described. Rather, the sentence describes a transition into a particular state of being. (After a number of comments, perhaps it's just best to jump to the end of this post where I walk through the traditional Japanese grammatical way of thinking ...