Más vale que guardes el alargador, no sea que haya otro apagón esta noche.

Más vale que is a common idiomatic way to give advice or a warning. In this sentence, it means something like you’d better.

So:

Más vale que guardes el alargador = You’d better put away the extension cord.

Literally, más vale comes from it is worth more / it is better, but in real use it often sounds like practical advice, sometimes with a slight warning tone.

Because más vale que is followed by the subjunctive.

Guardes is the present subjunctive of guardar for tú.

This happens because más vale que does not present a fact; it expresses advice, recommendation, or necessity. Spanish usually uses the subjunctive after expressions like that.

Compare:

• Guardas el alargador. = You put away the extension cord.
  
  • factual / indicative
• Más vale que guardes el alargador. = You’d better put away the extension cord.
  
  • advice / subjunctive

Here guardar means to put away, to store, or to put somewhere safe.

It does not mean to guard in the English sense of protecting something like a security guard. That English-looking connection can be misleading.

In context:

• guardar la ropa = to put away clothes
• guardar el móvil = to put away your phone
• guardar el alargador = to put away the extension cord

So the idea is probably to tidy it away or keep it ready/safe before a possible blackout.

In Spain, alargador usually means an extension cord.

A few useful notes:

• el alargador = extension cord
• un cable alargador = extension cable
• una regleta = power strip, usually with several sockets

So alargador is not exactly the same as regleta, although in casual speech people may sometimes blur the distinction depending on the object they mean.

Also note the gender: it is masculine here, so el alargador.

No sea que means something like:

• in case
• lest
• for fear that
• so that it doesn’t happen that

In this sentence, it introduces a possible undesirable situation:

no sea que haya otro apagón esta noche
= in case there’s another blackout tonight
or more literally
= lest there be another blackout tonight

It often has a slightly cautious or warning-like tone.

Because no sea que is a fixed expression that uses the subjunctive.

It comes from the verb ser, but here it is not functioning as a normal literal sentence like no es que.... Instead, no sea que acts as an idiomatic connector meaning in case / lest.

So learners should usually remember it as a chunk:

no sea que + subjunctive

For example:

• Llévate un paraguas, no sea que llueva.
• Cierra la puerta, no sea que entre el gato.

Because no sea que is followed by the subjunctive, and haya is the present subjunctive of haber.

Here haber is being used in its existential meaning, like English there is / there are:

• hay un apagón = there is a blackout
• haya otro apagón = there may be / there might be another blackout

Since the blackout is only a possible future event, Spanish uses the subjunctive.

Because Spanish uses haber to express existence:

• Hay un problema. = There is a problem.
• Puede que haya un problema. = There may be a problem.
• No sea que haya otro apagón. = In case there is another blackout.

So haya otro apagón literally means there be another blackout.

This is different from English, where we use there is rather than a special existential verb form.

Because otro normally goes directly before the noun without un.

So:

• otro apagón = another blackout
• otra vez = another time / again
• otro día = another day

Spanish usually does not say un otro in this meaning.

A different pattern is el otro = the other, as in:

• el otro día = the other day

But here the meaning is another blackout, so otro apagón is correct.

Apagón means a blackout or power outage, especially a noticeable failure of electricity.

In Spain, corte de luz is also very common and often sounds very natural in everyday speech.

The difference is often just one of nuance:

• apagón can sound a bit more like a blackout event
• corte de luz is a straightforward power cut

In many contexts, both would work.

Because in Spanish, after many expressions that trigger the subjunctive, you do not use the future tense in the subordinate clause. You often use the present subjunctive to refer to a future possibility.

So:

• no sea que haya otro apagón esta noche

refers to a possible event later tonight, even though haya is formally a present subjunctive form.

This is very normal in Spanish.

Yes. A very natural everyday version would be:

Guarda el alargador, por si hay otro apagón esta noche.

That means roughly the same thing: Put away the extension cord, in case there’s another blackout tonight.

The difference is mainly tone:

• no sea que haya... = a bit more cautious, warning-like, sometimes slightly more formal or literary
• por si hay... = very common, neutral, conversational

So the original sentence is correct and natural, but por si is probably what many people would say in relaxed everyday speech in Spain.

Usually, yes.

Compare:

• Mejor guarda el alargador. = Better put away the extension cord.
• Más vale que guardes el alargador. = You’d better put away the extension cord.

Más vale que... often sounds a little stronger, more cautionary, or more insistent. It can imply this is the sensible thing to do or you should really do this.

So in your sentence, it fits well with the second half, which mentions the risk of another blackout.