Mi madre dice que, si vuelve a haber otro apagón, habrá que desconectar la regleta y revisar el alargador con calma.

Because the speaker is reporting what the mother says in general or is saying now, not just what she said at one finished moment in the past.

• Mi madre dice que... = My mother says that...
• Mi madre dijo que... = My mother said that...

So dice keeps the sentence in the present.

It means if there is / if there happens to be again.

This part has two pieces:

• volver a + infinitive = to do something again
• haber = to be / to exist in expressions like hay (there is / there are)

So:

• haber otro apagón = for there to be another blackout
• volver a haber otro apagón = for there to be another blackout again

A natural translation is:

• if there’s another power cut
• if another blackout happens again

Because after si meaning if, Spanish normally uses the present indicative when talking about a real possible future condition.

So Spanish says:

• si vuelve a haber otro apagón... = if there’s another blackout again...

Not:

• si vuelva...

This is a very common point for English speakers, because they often expect the subjunctive in future-related clauses. But with si for real conditions, Spanish usually uses:

• si + present indicative for a possible future situation
• main clause often in the future, imperative, or similar

Examples:

• Si llueve, nos quedaremos en casa.
• Si tienes tiempo, llámame.

Because Spanish uses haber in impersonal expressions meaning there is / there are.

You already know:

• hay un problema = there is a problem
• hubo un apagón = there was a blackout

In the infinitive, that same idea is expressed with haber:

• puede haber un problema = there may be a problem
• va a haber tormenta = there is going to be a storm
• vuelve a haber otro apagón = there is another blackout again

So haber is the correct verb because the sentence is about the existence/occurrence of another blackout.

Apagón usually means a power cut, blackout, or power outage.

In Spain, apagón is a very natural word for electricity failing in an area or building.

Related words:

• apagón = blackout / power cut
• corte de luz = power cut
• corte eléctrico = electrical outage

Habrá que is an impersonal obligation structure. It means something like:

• it will be necessary to
• we’ll have to
• one will have to

So:

• habrá que desconectar la regleta
  = we’ll have to disconnect the power strip = literally it will be necessary to disconnect the power strip

This structure comes from:

• hay que = you have to / we have to / one has to
• habrá que = will have to

It does not name a specific subject. That is why it feels broader and more impersonal than tendremos que (we will have to).

Both are possible, but they are not exactly the same.

• habrá que = impersonal, more general
• tendremos que = we will have to, clearly names we

So this sentence sounds like practical advice or a general course of action:

• habrá que desconectar la regleta...
  = we’ll have to disconnect the power strip... / it’ll be necessary to disconnect...

It can sound slightly less direct and more neutral than tendremos que.

Desconectar means to disconnect / unplug / cut off from the power supply.

Apagar means to switch off / turn off.

That difference matters here:

• apagar una lámpara = turn off a lamp
• desconectar la regleta = disconnect/unplug the power strip

Since a regleta is something connected to the electricity supply, desconectar is the more precise choice.

In Spain:

• regleta usually means a power strip or multi-socket extension block
• alargador usually means an extension lead / extension cord

So the sentence suggests checking two related but different things:

• the power strip
• the extension cable/lead

There can be some overlap in everyday speech, but this is the usual distinction.

Because Spanish often uses the definite article where English might also use the, especially when talking about specific known objects in the situation.

Here, the speaker probably means the particular power strip and extension lead in the house.

So:

• la regleta = the power strip
• el alargador = the extension lead

This sounds natural if both speaker and listener already know which items are meant.

Con calma means calmly, carefully, or without rushing.

In this sentence, it suggests:

• checking the extension lead slowly
• not doing it in a panic
• taking the time to inspect it properly

So revisar el alargador con calma is something like:

• check the extension lead carefully
• inspect the extension cord without rushing

Because it interrupts the main clause.

The main structure is:

• Mi madre dice que ... habrá que desconectar la regleta y revisar el alargador con calma.

Inside that, the condition si vuelve a haber otro apagón is inserted in the middle, so commas help mark that interruption.

Compare:

• Mi madre dice que habrá que desconectar la regleta...
• Mi madre dice que, si vuelve a haber otro apagón, habrá que desconectar la regleta...

The commas make the sentence easier to read and show where the inserted condition begins and ends.

Not usually. Here revisar means:

• to check
• to inspect
• to examine

So:

• revisar el alargador = check the extension lead

This is a common false friend for English speakers, because revise in English usually means study again or edit something, while Spanish revisar often means inspect/check.

A few natural translations are:

• My mother says that if there’s another power cut, we’ll have to unplug the power strip and check the extension lead carefully.
• My mother says that if there’s another blackout, we’ll need to disconnect the power strip and inspect the extension cord carefully.

The Spanish is slightly impersonal because of habrá que, so a very close version would be:

• My mother says that if there’s another power cut, it’ll be necessary to disconnect the power strip and check the extension lead carefully.

But in normal English, we’ll have to sounds more natural.