Vi tänder två stearinljus när strömmen försvinner.

The verb tända means to light or to ignite.

So Vi tänder två stearinljus means that we light the candles, not just that we switch something on.

This verb is commonly used with things like:

• tända ett ljus = light a candle
• tända en cigarett = light a cigarette
• tända en eld = light a fire

For electrical devices, Swedish more often uses verbs like slå på or sätta på:

• slå på tv:n = turn on the TV

So tänder is the natural choice for candles.

Because stearinljus has the same form in singular and plural.

• ett stearinljus = a candle
• två stearinljus = two candles

This happens because the main noun here is ljus, and ljus is one of those Swedish nouns whose plural is identical to the singular.

Also, after a number like två, Swedish uses the indefinite plural, not the definite form.

So:

• två stearinljus = two candles
• de två stearinljusen = the two candles

Yes, very often you could just say ljus.

Stearinljus literally means a stearin candle, but in many everyday situations English speakers can think of it simply as candle.

Compare:

• ett ljus = a candle / a light
• ett stearinljus = a stearin candle

The shorter word ljus is very common, but it can also mean light, so the compound stearinljus is more specific.

Here strömmen means the power supply / the electricity.

In Swedish, this idea is usually expressed with the definite form:

• strömmen går = the power goes out
• strömmen försvinner = the power disappears / the power goes out

Using just ström would usually sound less natural in this context.

Also, ström can mean other things too, such as:

• electric current
• a stream / flow

So strömmen is the natural form here when talking about household power.

Yes, it works. Försvinna means to disappear, so strömmen försvinner means the power disappears.

That said, a very common everyday expression is:

• strömmen går = the power goes out

So both are understandable, but strömmen går may sound a bit more idiomatic in casual speech.

Because när means when, while om means if.

Use när when the event is treated as something that happens or is expected to happen:

• När strömmen försvinner, tänder vi två stearinljus.
• When the power goes out, we light two candles.

Use om when the event is uncertain or conditional:

• Om strömmen försvinner, tänder vi två stearinljus.
• If the power goes out, we light two candles.

In this sentence, när gives more of a whenever this happens feeling.

Because Swedish often uses the present tense for:

• habitual actions
• general truths
• things that happen whenever a situation occurs

So Vi tänder två stearinljus när strömmen försvinner does not have to mean it is happening right now. It can mean:

• this is what we usually do
• this is what we do whenever that happens

This is very natural in Swedish.

Yes, absolutely.

That version means the same thing, but it starts with the time clause for emphasis:

• Vi tänder två stearinljus när strömmen försvinner.
• När strömmen försvinner tänder vi två stearinljus.

Notice the word order in the second version:

• När strömmen försvinner, tänder vi...

When a subordinate clause comes first, the main clause still follows the Swedish verb-second pattern, so tänder comes before vi.

Två means exactly two.

If you want a looser meaning like a couple of or some, Swedish would usually use something else, for example:

• ett par stearinljus = a couple of candles
• några stearinljus = some candles

So in your sentence, två stearinljus is specific: exactly two candles.