Breakdown of Они пошли в подвал, в котором было очень темно.
они
they
очень
very
в
in / to (direction)
быть
to be
Questions & Answers about Они пошли в подвал, в котором было очень темно.
Why is котором used here instead of another form of который?
котором refers back to the masculine noun подвал (basement). Because the sentence describes a static location ('in which it was dark'), it requires the preposition в plus the Prepositional case. The masculine Prepositional form of который is котором.
Why does the first в take the Accusative case (подвал) while the second в takes the Prepositional case (котором)?
This sentence perfectly shows the difference between motion and location! The first part is about moving towards a destination ('went into'), so в takes the Accusative case. The second part describes a static state inside that destination ('in which it was dark'), so в switches to the Prepositional case.
Why is the neuter past tense было used for 'it was'?
In Russian, when you describe the general state of an environment, the weather, or a feeling (like 'it was dark', 'it is cold'), you use an impersonal sentence. Because there is no masculine or feminine subject performing the action, the past tense defaults to the neuter form было.
Why use the perfective verb пошли instead of the imperfective шли?