כשיש חושך בסלון, אני מדליק את האור.

כשיש is made of כש־ + יש.

• כש־ means when.
• יש means there is / there are.

So literally, כשיש חושך is when there is darkness.

In modern Hebrew, little connecting words like כש־ are often attached directly to the next word, so כשיש is completely normal.

יש is the Hebrew word used for existence: there is / there are.

So:

• יש חושך = literally there is darkness
• idiomatically = it is dark

English often uses it is with weather, time, and conditions, but Hebrew often uses יש for this kind of idea.

Good question. These are related, but they work differently:

• חושך = darkness (a noun)
• חשוך = dark (an adjective)

So the sentence says:

• כשיש חושך בסלון = when there is darkness in the living room

But Hebrew could also say:

• כשהסלון חשוך = when the living room is dark

Both are natural. The given sentence uses the there is darkness structure instead of describing the room directly with an adjective.

Because חושך here is being used as a general condition: darkness.

After יש, Hebrew very often uses a noun without the definite article:

• יש אור = there is light
• יש בעיה = there is a problem
• יש חושך = there is darkness / it is dark

Using החושך here would sound wrong in this structure.

In normal unpointed Hebrew writing, בסלון can represent either:

• בְּסָלוֹן = in a living room
• בַּסָּלוֹן = in the living room

In real life, context usually tells you which is meant. In a sentence like this, it will normally be understood as in the living room.

So this is one of those places where written Hebrew can be a little ambiguous unless vowel marks are added.

Hebrew often can omit subject pronouns, but here אני is helpful because מדליק by itself only tells you masculine singular, not the person.

So מדליק could mean:

• I turn on
• he turns on

The pronoun אני makes it clear that the subject is I.

In context, Hebrew speakers sometimes do drop the pronoun, but including it is very normal.

Because Hebrew present tense is often used for habitual actions too.

So:

• אני מדליק את האור can mean I am turning on the light
• but it can also mean I turn on the light / I usually turn on the light

With כש־ at the beginning, the sentence naturally gets a repeated or habitual meaning:

• כשיש חושך בסלון, אני מדליק את האור
  = When it’s dark in the living room, I turn on the light

The verb is להדליק, which means to light or to turn on.

For lights, candles, and many electrical things, להדליק is the standard verb:

• להדליק את האור = to turn on the light
• להדליק נר = to light a candle

The opposite is לכבות:

• לכבות את האור = to turn off the light

So מדליק is exactly the natural verb you would expect here.

Because מדליק is the masculine singular present-tense form.

If the speaker is male, you use:

• אני מדליק

If the speaker is female, you use:

• אני מדליקה

So the sentence changes depending on who is speaking.

את is the marker of a definite direct object.

Here, האור is the thing being turned on, and it is definite, so Hebrew uses את:

• אני מדליק את האור

This את does not mean you here. It has no direct English translation.

A very important rule:

• before a definite direct object: use את
• before an indefinite direct object: usually no את

Compare:

• אני מדליק את האור = I turn on the light
• אני מדליק אור = I turn on a light / light

Because the sentence is talking about a specific, understood light: the one in the living room.

That is why Hebrew uses the definite article:

• האור = the light

If you said just אור, it would sound more general or less specific. In this situation, את האור is the most natural choice, just like English usually says the light.

Yes. You could also say:

• אני מדליק את האור כשיש חושך בסלון

That is also correct.

The difference is mainly emphasis:

• כשיש חושך בסלון, אני מדליק את האור
  starts with the condition: when it’s dark...
• אני מדליק את האור כשיש חושך בסלון
  starts with the main action: I turn on the light...

When the כש־ clause comes first, a comma is very natural, as in the original sentence.