Перед выходом я всегда смотрю, выключен ли свет.

Because the preposition перед usually requires the instrumental case when it means before / in front of.

So:

• выход = exit, leaving
• перед выходом = before leaving / before going out

The form выходом is the instrumental singular of выход.

This is a very common pattern:

• перед работой = before work
• перед ужином = before dinner
• перед сном = before sleep / before bed

So перед выходом is a normal Russian way to say before leaving.

Here выход means going out / leaving, not the physical exit door.

So перед выходом is best understood as:

• before leaving
• before going out

Russian often uses a noun where English might prefer a verb phrase. Compare:

• Перед выходом я... = Before leaving, I...
• Перед отъездом я... = Before departing, I...

So even though выход can mean exit, in this sentence it refers to the action/event of leaving.

Yes, смотреть literally means to look / to watch, but in Russian it is also often used in the sense of:

• to check
• to see whether
• to make sure

So я смотрю, выключен ли свет means something like:

• I check whether the light is off
• I look to see if the light is off

This is very natural Russian.

A close English parallel is I look to see whether...

Because выключен ли свет is a subordinate clause: an embedded yes/no question, meaning whether the light is off.

Russian normally separates such clauses with a comma:

• Я знаю, что он дома.
• Я не помню, где он живёт.
• Я смотрю, выключен ли свет.

So the comma is there for the same reason English often uses whether to introduce a dependent clause.

Ли is a particle used to form an indirect yes/no question. It often corresponds to English whether or sometimes if.

So:

• выключен ли свет = whether the light is off

A very important point: ли usually comes right after the word that is being questioned or focused.

Here:

• выключен ли свет

This is literally something like:

• off whether the light

but naturally it means:

• whether the light is off

More examples:

• Я не знаю, придёт ли он. = I don’t know whether he will come.
• Спроси, дома ли она. = Ask whether she is at home.
• Интересно, будет ли дождь. = I wonder whether it will rain.

Because ли normally does not go at the very beginning of the clause.

In Russian indirect yes/no questions, ли usually follows the key word immediately. Here the key idea is выключен:

• выключен ли свет = whether the light is off

Compare:

• Я не знаю, дома ли он.
• Я спросил, придёт ли она.

So ли выключен свет sounds wrong in standard Russian.

Выключен is the short form, and that is the normal form used to describe a current state:

• Свет выключен. = The light is off.
• Дверь закрыта. = The door is closed.
• Окно открыто. = The window is open.

Выключенный is the full form, which behaves more like an adjective and is usually used before a noun:

• выключенный свет = the switched-off light
• выключенный компьютер = the switched-off computer

So in this sentence you need the short form:

• выключен ли свет = whether the light is off

Because in Russian, the verb to be is usually omitted in the present tense.

So:

• Свет выключен. literally = The light off/switched off
• natural English = The light is off.

This is completely normal in Russian.

Compare:

• Он дома. = He is at home.
• Она занята. = She is busy.
• Дверь открыта. = The door is open.

So выключен ли свет literally lacks is, but that is exactly how Russian works in the present tense.

Because the speaker is checking a state, not describing an action.

• выключен = is off / is switched off
  → a condition or result
• выключил = turned off
  → a completed action by someone
• выключаю = am turning off / turn off
  → an action in progress or habitual action

In this sentence, the person is checking the current situation:

• Is the light off?

So Russian uses the state form:

• выключен ли свет

not the action forms.

Yes. Both are possible.

• Перед выходом я всегда смотрю, выключен ли свет.
• Перед тем как выйти, я всегда смотрю, выключен ли свет.

The difference is mostly stylistic:

• перед выходом is more compact and noun-based
• перед тем как выйти is more explicitly verbal: before I go out / before leaving

Russian often prefers the shorter noun phrase when it sounds natural, so перед выходом is very idiomatic.

In this context, свет usually means the light in the sense of the room light / lights being on or off.

So выключен ли свет means:

• whether the light is off
• whether the lights have been turned off

Russian often uses singular свет where English may use either the light or the lights, depending on context.

It does not usually mean electricity here in the technical sense of power supply, although in other contexts свет can be used more broadly.

Not completely. Russian word order is fairly flexible, and всегда can move around:

• Я всегда смотрю, выключен ли свет.
• Я смотрю, выключен ли свет, всегда перед выходом.
• Перед выходом я всегда смотрю...

But the original order is the most natural and neutral.

Placing всегда before смотрю clearly shows that this is a habitual action:

• I always check...

So the sentence sounds natural and standard as written.