Es posible que el detector de humo no funcione.

Because es posible que normally triggers the subjunctive in Spanish.

• funciona = present indicative
• funcione = present subjunctive

Spanish uses the subjunctive after expressions of possibility, doubt, emotion, or uncertainty. Since es posible que means the speaker is not stating a fact, but a possibility, funcione is the correct form.

So:

• Es posible que el detector de humo no funcione. = It’s possible that the smoke detector may not work.
• El detector de humo no funciona. = The smoke detector doesn’t work.
  This sounds much more definite.

It is the present subjunctive of funcionar.

In this sentence, the present subjunctive does not mean “present time only.” It often refers to a situation that is true now or possibly in the future.

So no funcione can mean:

• it may not be working now
• it may not work when needed

That is very common in Spanish.

Here, que introduces the second clause: el detector de humo no funcione.

Spanish usually needs que after expressions like:

• es posible que
• es importante que
• es normal que
• es mejor que

So the structure is:

• Es posible que + clause

Without que, the sentence would be ungrammatical in this form.

Because in Spanish, no usually goes directly before the conjugated verb.

So:

• no funcione = may not work

This is the normal position for negation in Spanish.

Compare:

• No funciona. = It doesn’t work.
• Es posible que no funcione. = It may not work.

Spanish uses articles more often than English does.

Here, el suggests a specific smoke detector: the one in the house, room, building, etc.

• el detector de humo = the smoke detector

If you said just detector de humo, it would usually sound incomplete in this sentence.

Yes. Detector de humo is a normal and clear translation of smoke detector.

You may also hear:

• alarma de humo

But that can sometimes refer more to the alarm system/device in a general way, while detector de humo directly means the detector itself.

In most learning contexts, detector de humo is a very good standard option.

Yes. That is very natural too.

Both mean roughly the same thing:

• Es posible que el detector de humo no funcione.
• Puede que el detector de humo no funcione.

Both are followed by the subjunctive.

A rough difference in feel:

• Es posible que... = slightly more neutral or formal
• Puede que... = very common in everyday speech

Normally, no.

After es posible que, standard Spanish uses the subjunctive, so funcione is expected.

So this is correct:

• Es posible que el detector de humo no funcione.

Using funciona after es posible que would sound ungrammatical to most native speakers.

Here, funcionar means to work, to operate, or to function.

It is commonly used for machines, devices, systems, and technology:

• El ordenador no funciona. = The computer doesn’t work.
• La luz no funciona. = The light doesn’t work.
• El detector de humo no funciona. = The smoke detector doesn’t work.

So in this sentence, it means the device may not be operating properly.

Because es posible is an impersonal expression.

The word es does not agree with el detector. It is not saying the detector is possible. Instead, it means it is possible that...

So the structure is:

• Es posible que + clause

Other similar impersonal expressions are:

• Es importante que...
• Es necesario que...
• Es extraño que...

The es stays singular because the whole expression is fixed and impersonal.

Just a possibility.

That is the key nuance of the sentence. The speaker is not sure. They are saying there is a chance the smoke detector does not work.

If they were sure, they would be more likely to say:

• El detector de humo no funciona. = The smoke detector doesn’t work.

So the subjunctive helps show uncertainty.

In Spain, a simple pronunciation guide is:

• fun-see-OH-neh

More precisely:

• fun as in foon but shorter
• cio sounds roughly like thyo in much of Spain, or syo in many other accents
• stress on o: fun-cio-NE

So in much of Spain:

• funcione ≈ foon-thyo-NE

And in many Latin American accents:

• funcione ≈ foon-syo-NE