Uno de los faros no funciona bien desde ayer.

Because uno de los faros means one of the headlights / one of the lighthouses.

• un faro = a headlight / a lighthouse
• uno de los faros = one of the headlights / one of the lighthouses

You use this structure when something belongs to a group:

• uno de los libros = one of the books
• uno de mis amigos = one of my friends

Here, the speaker is talking about one item out of several.

Because uno is a pronoun here, not just an article.

Compare:

• un faro = a headlight / a lighthouse
  
  • Here un is an article before a noun.
• uno de los faros = one of the headlights
  
  • Here uno stands on its own and means one.

So in uno de los faros, uno is functioning like the English word one.

Because the subject is uno, not faros.

The sentence is built around:

• Uno ... no funciona bien = One ... isn’t working well

Even though faros is plural inside the phrase de los faros, the main subject is still singular: uno.

That is why the verb is singular:

• Uno de los faros no funciona bien.
• not Uno de los faros no funcionan bien.

In Spanish, no normally goes directly before the conjugated verb.

So:

• no funciona = does not work / isn’t working

This is the standard pattern:

• No tengo tiempo. = I don’t have time.
• No habla español. = He/She doesn’t speak Spanish.
• Uno de los faros no funciona bien.

This is different from English, where we often use do/does not.

Because bien is an adverb, and adverbs modify verbs.

• funciona is a verb
• so you need bien = well

Compare:

• funciona bien = works well
• es bueno = is good

So:

• El coche funciona bien. = The car works well.
• El coche es bueno. = The car is good.

In this sentence, the idea is about how it works, so bien is the correct word.

Here funcionar means to work in the sense of to operate properly.

It is commonly used for machines, devices, systems, parts of vehicles, etc.

Examples:

• El móvil no funciona. = The phone isn’t working.
• La calefacción funciona bien. = The heating works well.
• Uno de los faros no funciona bien. = One of the headlights isn’t working properly.

So this is not work in the sense of having a job. It means something is operating correctly or incorrectly.

Because Spanish often uses the present tense for something that started in the past and is still true now.

• desde ayer = since yesterday
• If it has been happening since yesterday and continues now, Spanish commonly uses the present.

So:

• Uno de los faros no funciona bien desde ayer.

This is like:

• Vivo aquí desde 2020. = I have lived here since 2020.
• literally in Spanish: I live here since 2020

This is very normal Spanish usage.

Desde ayer means since yesterday.

It gives the starting point of a situation that continues up to now.

• desde = since / from
• ayer = yesterday

So the idea is: the problem began yesterday and is still happening now.

A useful contrast:

• desde ayer = since yesterday
• desde hace un día = for a day / for one day

Both can refer to duration up to the present, but desde ayer focuses on the starting point.

Yes. In Spanish, faro can mean:

• lighthouse
• headlight

In everyday car-related language, los faros often means the headlights.

For example:

• Encendí los faros. = I turned on the headlights.
• Se ha fundido un faro. = One headlight has blown.

Usually the context makes the meaning clear. In a sentence like Uno de los faros no funciona bien desde ayer, most people would probably understand headlights, especially if the topic is a car.

Yes, that is also natural.

There is a small nuance:

• no funciona bien = doesn’t work well
• funciona mal = works badly / isn’t working properly

In many situations, they mean almost the same thing.

Sometimes funciona mal can sound a bit more direct or stronger, while no funciona bien can sound slightly softer, but both are common and correct.

Yes, you could, and it is grammatically correct.

But in Spanish, especially for general ongoing situations, the simple present is often preferred:

• no funciona bien desde ayer

Using está funcionando may sound a bit more immediate or descriptive, as if you are focusing on the ongoing malfunction right now.

In many everyday contexts, Spanish speakers would naturally choose the simpler version:

• Uno de los faros no funciona bien desde ayer.

Yes. Spanish word order is flexible, so you can move desde ayer for emphasis.

For example:

• Uno de los faros no funciona bien desde ayer.
• Desde ayer, uno de los faros no funciona bien.

Both are correct.

The second version puts more focus on since yesterday. The first version is probably the more neutral, straightforward order.

A simple pronunciation guide:

• faros ≈ FA-rohs
• funciona ≈ foon-THYO-na in much of Spain

Notes for Spain:

• The c before i in funciona is pronounced like the th in think in most of Spain.
• In many parts of Latin America, it would sound more like s: foon-SYO-na.

Also:

• r in faro is a tapped Spanish r, not a strong English r
• stress:
  • FA-ros
  • fun-CIO-na (stress on cio)