Uma dobradiça do armário faz barulho, porque falta um parafuso.

Because dobradiça is a feminine singular noun, so the indefinite article is uma.

• uma dobradiça = a hinge / one hinge
• a dobradiça = the hinge

In this sentence, uma suggests that the speaker is referring to one hinge of the cupboard, not necessarily identifying it as a specific one already under discussion.

Do is the contraction of de + o.

• de = of / from
• o = the, for a masculine singular noun
• de + o = do

So do armário literally means of the cupboard/wardrobe/cabinet.

Because armário is masculine singular, you get:

• do armário not
• da armário

Yes. Armário can be translated in different ways depending on context, such as:

• cupboard
• cabinet
• wardrobe
• sometimes closet

So uma dobradiça do armário could be a hinge on the cupboard, a hinge on the cabinet, or a hinge on the wardrobe, depending on what kind of furniture is meant.

Because fazer barulho is a very common expression meaning to make noise.

So:

• faz barulho = makes noise
• in natural English here, often is noisy or is making a noise
• for a hinge specifically, English might also say squeaks or creaks

Portuguese often uses fazer + noun in fixed expressions, and fazer barulho is one of them.

In expressions like fazer barulho, Portuguese usually uses the noun without an article when speaking in a general sense.

So:

• faz barulho = makes noise / is noisy
• faz um barulho = makes a noise, often referring to a particular noise or emphasizing it

This is similar to how some expressions become fixed as a unit. Compare:

• fazer barulho
• fazer sentido
• ter fome
• ter pressa

Here porque means because, so it introduces the reason.

Very roughly:

• porque = because
• por que = why / for which reason in certain structures
• porquê = the reason, used as a noun

In this sentence, the second clause gives the explanation, so porque is the correct form.

Here faltar means to be missing or to be lacking.

So falta um parafuso is literally something like:

• a screw is missing
• one screw is missing

This is an important pattern in Portuguese: the thing that is absent is often the grammatical subject of faltar.

The subject is um parafuso.

That may feel backward to an English speaker, because English often says something like it is missing a screw. But Portuguese structures it differently:

• Falta um parafuso. = A screw is missing.
• Faltam dois parafusos. = Two screws are missing.

So the verb agrees with the missing thing.

Because the subject, um parafuso, is singular.

• falta um parafuso = one screw is missing
• faltam dois parafusos = two screws are missing

So the verb changes according to the number of missing items.

Not explicitly by itself, but the context makes that clear.

In the sentence, the listener naturally understands that the missing screw belongs to the hinge, because that is the most logical connection.

If you wanted to make it fully explicit, Portuguese could say:

• À dobradiça falta um parafuso.
• Falta um parafuso à dobradiça.
• Falta-lhe um parafuso.

All of these mean more clearly that the hinge is missing a screw.

Yes, especially in European Portuguese.

• faz barulho can describe a general or current situation
• está a fazer barulho emphasizes that it is making noise right now
• falta um parafuso states the fact that a screw is missing
• está a faltar um parafuso can sound a bit more immediate or situational

For Portugal, estar a + infinitive is the normal progressive pattern:

• está a fazer
• está a faltar

A form like está faltando is associated with Brazilian Portuguese, not standard European Portuguese usage.

A few useful pronunciation points:

• dobradiça: the ç sounds like s
• barulho: the lh is a palatal sound, similar to the lli in some pronunciations of million, but not exactly the same
• armário: the stress is on má
• in European Portuguese, unstressed vowels are often reduced, so words can sound shorter and less open than an English speaker expects

A rough learner-friendly guide:

• dobradiça ≈ do-bra-DI-sa
• barulho ≈ ba-RU-lyu

Those are only approximations, but they help with the main sounds.

You would change the verb and the noun to the plural:

• Uma dobradiça do armário faz barulho, porque faltam dois parafusos.

The key change is:

• falta um parafuso
• faltam dois parafusos

Again, the verb agrees with the thing that is missing.