Kun sähkö katkeaa, lamppu sammuu heti.

Here kun means when.

It introduces a time clause:

• Kun sähkö katkeaa = When the electricity goes out / when the power is cut
• lamppu sammuu heti = the lamp goes out immediately

In Finnish, kun is very common for when in statements about something that happens whenever another thing happens.

Finnish often uses the present tense for:

• general truths
• repeated events
• future situations that are understood from context

So Kun sähkö katkeaa, lamppu sammuu heti can mean:

• When the electricity goes out, the lamp goes out immediately
• If the power goes out, the lamp goes out immediately
• When the power goes out next time, the lamp will go out immediately

This is very natural in Finnish. English often prefers future forms in some contexts, but Finnish usually does not need them here.

Literally, sähkö katkeaa is something like the electricity breaks off or the power gets cut.

The verb katketa means to break, to snap, or to be interrupted. With sähkö, it means that the electric supply stops.

So natural English translations are:

• the electricity goes out
• the power cuts out
• the power is interrupted

It may sound a bit unusual if translated word for word, but it is normal Finnish.

Katketa is the infinitive, the basic dictionary form: to be cut off / to break.

In the sentence, you need a finite verb, not the infinitive.
Katkeaa is the 3rd person singular present tense form:

• minä katkean
• sinä katkeat
• hän/se katkeaa

Here the subject is sähkö, which is singular, so the verb is katkeaa.

This is an important Finnish verb pair:

• sammua = to go out, to switch off by itself, to become extinguished
• sammuttaa = to switch something off, to extinguish something

So:

• Lamppu sammuu = The lamp goes out
• Sammutan lampun = I switch off the lamp

In your sentence, the lamp is not actively switched off by a person. It goes out because the electricity stops, so sammuu is the correct verb.

Because lamppu is the subject of the main clause, and subjects are normally in the nominative case.

• lamppu = nominative, basic form
• lampun = genitive/accusative form, often meaning of the lamp or used as an object in some contexts

Here the lamp is the thing doing the action of going out, so it stays in the nominative:

• lamppu sammuu = the lamp goes out

Heti means immediately, right away.

It is placed after the verb here:

• lamppu sammuu heti = the lamp goes out immediately

That word order is very natural. Finnish word order is fairly flexible, but this version is neutral and clear.

You could move heti for emphasis in some contexts, but the original sentence is the most straightforward form.

Because Kun sähkö katkeaa is a subordinate clause, and it comes before the main clause.

So the sentence structure is:

• subordinate clause: Kun sähkö katkeaa
• main clause: lamppu sammuu heti

Finnish normally uses a comma between them in this situation.

Yes, very often it could.

Although kun basically means when, in practical use a sentence like this can sound close to English if when it describes what happens in that situation.

So depending on context, this sentence may feel like:

• When the power goes out, the lamp goes out immediately
• If the power goes out, the lamp goes out immediately

The Finnish sentence itself is still built with kun.

Finnish has no articles like English a/an and the.

So:

• sähkö can mean electricity, the electricity, or sometimes power
• lamppu can mean a lamp or the lamp

The exact meaning comes from context.

In this sentence, English usually uses the because the situation feels specific or generic in a definite way:

• When the electricity goes out, the lamp goes out immediately

But Finnish does not need a separate word for that.

Yes, sähkö is singular here.

Finnish can also use sähköt in everyday language to mean the electricity supply / the power, especially in expressions like:

• Sähköt katkesivat = The power went out

So both singular and plural-style expressions can appear in real Finnish, depending on phrasing.
In your sentence, sähkö katkeaa is completely correct and understandable.

It most naturally sounds like a general rule or a typical result:

• whenever the electricity goes out, the lamp goes out immediately

But depending on context, it could also refer to a specific future event:

• When the power goes out, the lamp will go out immediately

Finnish present tense is often broad enough to cover both meanings.

Yes, Finnish word order is flexible, but different orders can sound more marked or emphasize different parts.

The neutral version is:

• Kun sähkö katkeaa, lamppu sammuu heti.

You could also say:

• Lamppu sammuu heti, kun sähkö katkeaa.

This means the same thing: The lamp goes out immediately when the electricity goes out.

Both are natural. The original version starts with the time condition first, which is very common.