Strömmen är fortfarande borta.

Because strömmen is the definite singular form of ström.

• ström = power/current/flow
• strömmen = the power/current

In Swedish, the definite article is often added as a suffix instead of being a separate word. So where English says the power, Swedish says strömmen.

The double m is just part of the spelling pattern when the ending is added.

Swedish usually puts the at the end of the noun.

So:

• en ström = a current / a power supply
• strömmen = the current / the power

That is why you do not need a separate word before the noun here.

Not literally in every context. Ström can mean several related things, such as:

• current
• flow
• stream
• electric power

In this sentence, strömmen means the electrical power supply, because that is the natural meaning in this context.

Yes. Elen är fortfarande borta is also very natural.

There is a small difference in feel:

• elen = electricity, the electric supply
• strömmen = the current/power

In everyday speech about outages, both can work. Elen is often the most straightforward word for utility electricity, while strömmen is also very common.

Fortfarande means still.

It shows that the situation has not changed. So the sentence says that the power was already gone before, and it remains gone now.

That is the normal word order in a Swedish main clause.

Here the order is:

• Strömmen = subject
• är = finite verb
• fortfarande = adverb
• borta = complement/predicative word

Swedish main clauses usually follow the V2 rule, meaning the finite verb comes in the second position.

Here borta means something like:

• gone
• away
• not there
• unavailable

With electricity, är borta is a natural way to say that the power is gone or out.

So strömmen är borta means the power is not available.

Swedish often describes this as a state:

• Strömmen är borta = the power is gone / out

English can do something similar with is out.

There are other possible Swedish expressions too, for example:

• Strömmen har gått = the power has gone out
• Det är strömavbrott = there is a power outage

But är borta is very natural and common.

In this kind of sentence, borta behaves as an indeclinable word, so it does not change form.

For example:

• Strömmen är borta
• Nyckeln är borta
• Pengarna är borta

You do not change borta for gender or number.

Är is the present tense of vara (to be).

Even though the situation began earlier, Swedish uses the present tense because it describes the current state:

• the power is still gone

That matches English quite closely here.

Sometimes yes, but the version in your sentence is the most neutral and standard:

• Strömmen är fortfarande borta.

You may also hear:

• Strömmen är borta fortfarande.

That is possible, but it sounds more marked or conversational. For a learner, the safest choice is to put fortfarande before borta.

It sounds neutral and everyday.

It is a completely natural thing to say in normal conversation if the electricity has not come back yet. It is neither especially formal nor especially slangy.