Learn Hardcore Russian: Я ищу выключатель, чтобы выключить свет перед сном. - I am looking for the light switch to turn off the light before sleep.

In Russian, a clause introduced by чтобы (meaning in order to / so that) is treated as a subordinate clause, so it is normally separated by a comma:
Я ищу выключатель, чтобы выключить свет...
Even though the verb after чтобы is an infinitive (not a finite verb), Russian punctuation still uses the comma here.

What exactly does чтобы + infinitive mean, and how is it different from just using an infinitive without чтобы?

чтобы + infinitive explicitly expresses purpose: in order to do X.

Я ищу выключатель, чтобы выключить свет. = I’m looking for the switch in order to turn off the light.
Without чтобы, Russian can still use an infinitive after certain verbs, but it often changes the structure or sounds less like a purpose clause. чтобы is the most direct way to mark purpose.

Why is выключить perfective, not выключать?

Because the goal is a single completed action: to turn the light off (once).

выключить (perfective) = to switch off (complete, one-time result)
выключать (imperfective) = to be switching off / to switch off regularly or as a process
In this context, you want the result (light ends up off), so выключить is natural.

Why is ищу imperfective? Would найду work?

искать / ищу is imperfective and describes an ongoing process: I’m looking (right now).
найти / найду is perfective and focuses on the result: I’ll find / I found.
Both can work, but they mean different things:

Я ищу выключатель... = I’m in the process of searching.
Я найду выключатель, чтобы выключить свет... = I’ll find the switch in order to turn off the light (more like a plan/promise).

What case is выключатель in, and why?

выключатель is in the accusative, because it’s the direct object of искать (to look for).
For an inanimate masculine noun like выключатель, the accusative form looks the same as the nominative: выключатель.

What case is свет in, and why?

свет is also accusative because it’s the direct object of выключить (to turn off).
Again, it’s inanimate masculine, so accusative = nominative in form: свет.

Does свет here mean “light” or “the lights/electricity”?

In this context, выключить свет is a very common set phrase meaning to turn off the light / the lights (i.e., the room light). It can refer to the light being on in the room, not “light” in an abstract physics sense.

Why is it перед сном and not перед сон or перед сна?

The preposition перед requires the instrumental case.
So сон becomes сном (instrumental singular):

перед сном = before sleep / before going to bed
This phrase is idiomatic and very common.

Is перед сном the same as “before going to bed”?

Often yes in meaning. перед сном is literally before sleep, but in everyday speech it commonly corresponds to before going to bed / before sleeping. If you want to be more explicit about bed, you can also say перед тем, как лечь спать (before lying down to sleep), but перед сном is simpler and natural.

Could the word order be changed? For example: Перед сном я ищу выключатель...

Yes. Russian word order is flexible and mainly changes emphasis:

Я ищу выключатель, чтобы выключить свет перед сном. (neutral: what you’re doing and why)
Перед сном я ищу выключатель, чтобы выключить свет. (emphasis: “before bed” is the setting/time)
Чтобы выключить свет перед сном, я ищу выключатель. (strong emphasis on purpose)

How do you pronounce выключатель and where is the stress?

выключа́тель has stress on -ча́-: vy-klyu-CHA-tel.
And вы́ключить has stress on the first syllable: вы́-klyu-chit.