Quiero el nuevo porque el otro no funciona bien.

Because in Spanish, adjectives like nuevo and words like otro can stand in for a noun when the noun is understood from context.

So el nuevo means something like:

• the new one
• the new one/item

And el otro means:

• the other one

The article el is what helps turn them into noun-like expressions.

Yes, but it is intentionally omitted because it is obvious from context.

For example, this sentence could really mean:

• Quiero el móvil nuevo porque el otro no funciona bien.
• Quiero el coche nuevo porque el otro no funciona bien.
• Quiero el ordenador nuevo porque el otro no funciona bien.

Spanish often leaves out the noun when both speakers already know what is being talked about. English does this too with the new one and the other one.

Because nuevo agrees in gender and number with the omitted noun.

If the hidden noun is masculine singular, you use el nuevo.

Examples:

• el móvil nuevo → quiero el nuevo
• el libro nuevo → quiero el nuevo

If the omitted noun were feminine singular, it would be la nueva:

• la chaqueta nueva → quiero la nueva

So the sentence tells you the omitted noun is probably masculine singular.

Otro by itself usually means another or other before a noun:

• otro coche = another car / other car

But when you mean the other one, Spanish normally uses the article:

• el otro = the other one
• la otra = the other one (feminine)

So in this sentence, el otro is correct because it refers to a specific alternative.

Because porque is the conjunction meaning because.

• porque = because
• por qué = why in questions

Examples:

• Lo quiero porque es útil. = I want it because it is useful.
• ¿Por qué lo quieres? = Why do you want it?

In your sentence, the speaker is giving a reason, so porque is needed.

Because the subject is el otro, which is singular: the other one.

So:

• el otro no funciona bien = singular
• los otros no funcionan bien = plural

The verb must agree with its subject, even when the noun itself is omitted.

Because funcionar is a verb, and bien is an adverb.

• bien modifies verbs
• bueno is an adjective and modifies nouns

So:

• no funciona bien = it does not work well

Compare:

• un aparato bueno = a good device
• el aparato funciona bien = the device works well

A very common learner mistake is trying to use bueno after a verb when Spanish needs bien.

Not literally. El otro means the other one.

However, in context, the other one may be the previous one, the older one, or the one currently being used. That is why in natural English it may sometimes be understood as the old one, but Spanish itself is not directly saying old.

If you specifically wanted the old one, you would more literally use something like:

• el viejo
• el antiguo

But those can carry different nuances, so el otro is often the more neutral choice.

Yes, a Spanish speaker in Spain could absolutely say Quiero el nuevo in everyday speech.

Quiero is direct, but not automatically rude. Whether it sounds polite depends a lot on tone and situation.

In a shop, for example:

• Quiero el nuevo = direct, natural, a bit blunt depending on tone
• Quiero el nuevo, por favor = more polite
• Me gustaría el nuevo = softer
• Prefiero el nuevo = I prefer the new one

So quiero is grammatically normal and commonly used, but if you want to sound more polite, there are softer options.

Yes, but it means something slightly different.

• Quiero el nuevo = I want the new one
  This suggests there are specific known options, and you want the new one among them.

• Quiero uno nuevo = I want a new one
  This focuses more on wanting a replacement that is new, not necessarily a specific known item.

So both are possible, but they are not identical.

It is the most natural straightforward order:

1. what the speaker wants: Quiero el nuevo
2. the reason: porque el otro no funciona bien

Spanish often follows this same logic as English.

You could change the order for emphasis:

• Porque el otro no funciona bien, quiero el nuevo.

This is still correct, but it sounds more marked or emphatic because it puts the reason first.

Usually it just means new in contrast to the other one.

Whether it means brand new depends on context.

For example:

• If you are choosing between two models, el nuevo may mean the newer one
• If something is broken and you need a replacement, el nuevo may imply a new replacement
• If you specifically want to stress brand new, Spanish might use:
  • completamente nuevo
  • nuevo de verdad
  • a estrenar in some contexts

So nuevo by itself is often enough, but context decides the exact nuance.

Yes, very commonly.

Funcionar bien is one of the most natural ways to say that a machine, device, system, or object is working properly.

Examples:

• El móvil no funciona bien.
• La tele no funciona bien.
• Este ordenador funciona muy bien.

It is a very useful everyday expression in Spain as well as in other Spanish-speaking countries.