El mecánico dijo que uno de los faros no funciona bien.

Because Spanish usually uses a definite article with professions or nouns when talking about a specific person already identified in the situation.

• el mecánico = the mechanic
• mecánico by itself would usually sound incomplete here

In this sentence, we are talking about a particular mechanic, so el is needed.

The accent mark shows where the stress goes: me-CÁ-ni-co.

Without the accent, Spanish stress rules would make you expect the stress somewhere else. The written accent helps you pronounce it correctly.

It is also part of the standard spelling of the word mecánico.

Dijo is the preterite form of decir, meaning said.

• dijo = he/she said
• infinitive: decir = to say / to tell

The preterite is used because this is a completed action in the past: the mechanic said it at some point.

A few related forms:

• dice = he/she says
• decía = he/she was saying / used to say
• dijo = he/she said

In Spanish, que often introduces a reported statement after verbs like decir, pensar, creer, and so on.

So:

• dijo que... = said that...

This works very much like English that in He said that...

In English, that is often omitted:

• The mechanic said one of the headlights...

In Spanish, que is normally kept:

• El mecánico dijo que...

Because uno here is a pronoun-like form meaning one, not the article a/an.

Compare:

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

You need uno because the structure is one of + plural noun:

• uno de los faros
• uno de los problemas
• uno de mis amigos

So un de los faros is not correct.

This is the normal Spanish way to say of the headlights.

The pattern is:

• uno de los + plural noun
• one of the + plural noun

Examples:

• uno de los faros = one of the headlights
• uno de los coches = one of the cars
• uno de los libros = one of the books

Los is used because faros is masculine plural.

Because the subject is uno, which is singular.

Even though faros is plural, the main subject of the sentence is one:

• uno de los faros = one of the headlights

So the verb must also be singular:

• uno ... funciona
• not uno ... funcionan

This is exactly like English:

• One of the headlights doesn’t work well
• not One of the headlights don’t work well

Because bien is an adverb, and it modifies the verb funciona.

• bien = well
• bueno = good

After a verb like funcionar, you need the adverb:

• funciona bien = works well

Compare:

• Es un faro bueno = It is a good headlight
• El faro funciona bien = The headlight works well

So bueno describes a noun, while bien describes how something works.

Yes, it can mean both, depending on context.

In general Spanish:

• faro can mean lighthouse
• in vehicle contexts, it can also mean headlight

In this sentence, because a mechanic is speaking, faros clearly means the car’s headlights.

In Spain, faro is a very common word for a vehicle headlight. You may also hear more specific terms depending on the type of light, but faro is completely natural here.

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

So:

• no funciona = does not work

This is the normal pattern:

• no sé = I don’t know
• no viene = he/she isn’t coming
• no funciona bien = it doesn’t work well

Yes. That would simply be a shorter sentence with the reported speech removed.

• El mecánico dijo que uno de los faros no funciona bien. = The mechanic said that one of the headlights isn’t working well.

• Uno de los faros no funciona bien. = One of the headlights isn’t working well.

So the second part is a complete statement by itself. The first sentence just adds who said it.

Spanish often uses the simple present where English might use either works or is working.

• no funciona bien can mean doesn’t work well or, depending on context, isn’t working properly

The simple present is very natural for describing how something functions in general or at the moment.

You could say no está funcionando bien, but that puts more emphasis on the current ongoing situation. In many everyday cases, no funciona bien is the more usual choice.

A simple pronunciation guide would be:

el meh-KAH-nee-ko DEE-ho keh OO-no deh los FA-ros noh foon-SYOH-nah BYEN

A few useful notes for Spain Spanish:

• c before i in mecánico sounds like the th in thin
• dijo has a soft h-like sound for the j
• ll does not appear here, so there is nothing special to watch for there
• bien is one syllable or very close to one syllable for many speakers

A more Spain-oriented approximation: el me-THA-ni-ko DI-ho ke U-no de los FA-ros no fun-THYO-na byen