Learn Hardcore Russian: Вечером в коридоре темно, поэтому я ищу выключатель. - In the evening it is dark in the corridor, so I am looking for the light switch.

Вечером is the instrumental case form of вечер, but in this use it functions as a time adverb meaning in the evening / in the evenings. Russian often uses the instrumental to express “at (a time of day/season)”: утром, днём, вечером, ночью, зимой, летом.

Does вечером mean “this evening” or “in the evening (generally)”?

It’s ambiguous without context. Вечером can mean:

(this) evening (a specific time, especially in a narrative)
in the evening as a general statement
If you want to clearly mean “in the evenings (habitually)”, Russian often uses по вечерам.

Why is it в коридоре (not в коридор)?

Because в + location (“in/inside”) takes the prepositional case: в коридоре = in the hallway.
В коридор would be accusative and would usually mean motion into the hallway (direction), e.g. я вошёл в коридор.

What part of speech is темно? Why not тёмный?

Темно is a “category of state” word (a predicative adverb) meaning it is dark. It’s used in impersonal sentences about conditions:

В комнате холодно.
На улице шумно.

Тёмный is an adjective and needs a noun to describe: тёмный коридор = a dark hallway (describing the hallway as an object).
Here the sentence describes the situation/state, so темно is natural.

Where is the verb “to be” in в коридоре темно?

Russian usually omits the present-tense form of “to be” (is/are) in statements like this.
So в коридоре темно literally works like in the hallway (it) dark, i.e. it’s dark in the hallway.

Why is there a comma before поэтому?

Поэтому means therefore/so, and it often introduces a result clause. In Russian it’s commonly separated by a comma, similar to English:

…, поэтому … = …, so … / …, therefore …

Can I replace поэтому with потому что?

Not directly, because they structure the logic differently:

темно, поэтому я ищу… = It’s dark, so I’m looking… (cause → result)
я ищу…, потому что темно = I’m looking…, because it’s dark (result → cause)

Both are correct; you just rearrange the clauses.

Why is я included? Can it be omitted?

Yes, it can often be omitted because the verb ending shows the person:

(Я) ищу выключатель. = (I) am looking for the switch.

You keep я for emphasis/contrast (e.g., “I’m the one looking”).

Why is ищу imperfective—should it be perfective?

Искать (imperfective) focuses on the process: I’m searching / looking. That fits the situation (you haven’t found it yet).
The perfective partner is commonly найти (to find): я найду выключатель = I’ll find the switch (focus on the result).

Why doesn’t выключатель change form? Isn’t it accusative?

It is accusative, but выключатель is an inanimate masculine noun, and for inanimate masculine nouns the accusative usually equals the nominative:

nominative: выключатель
accusative: выключатель

(With animate masculine nouns, accusative often matches genitive instead.)

Is the word order fixed? Could I say Я ищу выключатель, потому что вечером в коридоре темно?

Russian word order is flexible. Your version is grammatical and means the same basic thing. The original order (Вечером в коридоре темно, поэтому…) foregrounds the setting first (time + place + condition), then gives the consequence.

How do I pronounce and stress the key words?

вЕчером (stress on the first е)
в коридОре (stress on о)
темнО (stress on the last о)
поэтОму (stress on о)
ищУ (stress on у)
выключАтель (stress on а)