Если газ на кухне не выключен, может произойти утечка.

In Russian, a comma is normally used to separate a subordinate clause from the main clause.

Here:

• Если газ на кухне не выключен = If the gas in the kitchen is not turned off
• может произойти утечка = a leak may occur

So the sentence has the structure:

• Если ... , ... = If ..., ...

This is very similar to written English, where you also usually use a comma when the if-clause comes first.

Газ is the subject of the first clause, so it stays in the nominative singular:

• газ = gas

In Если газ на кухне не выключен, the basic idea is:

• газ ... не выключен = the gas is not turned off

Since газ is what is being described, it remains nominative.

Also, Russian does not use articles like a or the, so газ can mean gas or the gas, depending on context.

This is a very common learner question because English says in the kitchen, but Russian usually says на кухне.

For location, Russian often uses:

• на кухне = in the kitchen
• на работе = at work
• на почте = at the post office

So although на often means on, in many fixed expressions it corresponds to English in or at.

Here, на кухне is just the normal idiomatic way to say in the kitchen.

Выключен is a short-form passive participle, and in sentences like this it works much like an adjective meaning:

• switched off
• turned off

So:

• газ не выключен = the gas is not turned off / the gas has not been turned off

Compare:

• выключить = to turn off (infinitive)
• выключает = he/she/it turns off (present tense verb)
• выключен = turned off / is turned off (state/result)

Russian often uses this type of form to describe the current state of something after an action.

The ending shows agreement with газ.

• газ is masculine singular
• so the short-form participle is выключен

Other forms would be:

• выключена for feminine singular
• выключено for neuter singular
• выключены for plural

For example:

• лампа выключена = the lamp is switched off
• окно закрыто = the window is closed
• двери закрыты = the doors are closed

So выключен is masculine because it matches газ.

In Russian, negation with не usually goes directly before the word being negated.

Here:

• не выключен = not turned off

So:

• газ не выключен = the gas is not turned off

This is the normal and expected placement. Russian does not need an extra verb like English is in this kind of present-tense sentence, so не simply goes before выключен.

This structure means a leak may occur.

Breakdown:

• может = can / may
• произойти = to happen / to occur
• утечка = leak

Literally, it is something like:

• may occur a leak

But in natural English, we say:

• a leak may occur

Russian word order is more flexible than English, so the subject утечка can come after the verb phrase.

Because it follows the modal verb может.

In Russian, after verbs like мочь (to be able to / may / can), the next verb is usually in the infinitive:

• может произойти = may occur
• может случиться = may happen
• может начаться = may begin

So:

• может is the finite verb
• произойти stays in the infinitive

This is very similar to English may occur or can happen.

Утечка means a leak.

In this sentence, it most naturally means a gas leak. Russian does not have to repeat газовая here because the context already makes it clear.

You may also see:

• утечка газа = a gas leak
  literally leak of gas
• утечка воды = a water leak
• утечка информации = an information leak / data leak

Here, утечка is in the nominative singular because it is the subject of произойти.

Yes. Russian word order is flexible, and you could also say:

• Может произойти утечка, если газ на кухне не выключен.

This still means the same thing: A leak may occur if the gas in the kitchen is not turned off.

The difference is mainly one of emphasis and information flow:

• Если ... , ... puts the condition first
• ... , если ... puts the result first

Both are natural.

In everyday Russian, газ выключен / не выключен often refers to the gas being turned off or left on, especially in a household context.

So in this sentence, газ is understood practically, not chemically. It refers to the gas supply or burner being left on in the kitchen.

That is why the sentence warns about утечка. The meaning is based on real-world context, not just the dictionary meaning of gas.