Preterite vs. imperfect and "ser" vs. "estar" to describe a natural condition in the past

Question

How does one decide the correct verb and tense while describing a time or natural condition in the past? Take, for instance, the following sentence:

It was dark

Which of the following would be the most appropriate/natural translation and why?

Estaba oscuro

Estuvo oscuro

Era oscuro

Fue oscuro

My guess is it should be estar because it's a temporary situation and in the preterite form since it's one particular instance being described here (as against "It used to be dark"). Am I correct here?

Will the verb and tense change in any way if we said, "It was dark already," instead?

And lastly, what happens if we change the sentence into a progressive?

It was getting dark

Answer 1

Notice that in Spanish there is a verb form for the adjetive "oscuro", which is "oscurecer" which, among others, has the meaning of

Ir anocheciendo, faltar la luz y claridad desde que el Sol empieza a ocultarse

So you could also use the [haber + participio] to say

Había oscurecido.

Había llovido

For the progressive action

Estaba oscureciendo

Estaba lloviendo

You are right about the use of Estar for a natural condition in the past. About using pretérito imperfecto ("Estaba oscuro") or pretérito perfecto simple ("Estuvo oscuro") it all dependes on the continuity of the action (do you want to convey that it was dark only for some time? If not the most natural tense would be the imperfecto).

Answer 2

Usually background information requires the imperfect, so in a sentence like.

It was dark when John Doe arrived at the door.

It needs to be the imperfect.

Estaba oscura cuando fulano de tal llegó a la puerta.

If you wanted to say, "It was dark already", I'd just add ya to the sentence.

Ya Estaba oscura cuando fulano de tal llegó a la puerta.

For

It was getting dark.

Use the pronominal form oscurecerse

Estaba oscureciéndose
Se oscurecía