Si hubieras traído más clavos, el estante ya estaría terminado.

Because Spanish uses the pluperfect subjunctive after si for an unreal or impossible condition in the past.

So:

• si hubieras traído más clavos = if you had brought more nails

This is the standard pattern for past hypothetical situations:

• Si + pluperfect subjunctive, ... conditional

Examples:

• Si hubieras llamado, habría ido.
• Si lo hubiéramos sabido, no habríamos venido.

Two important contrasts:

• si habías traído is indicative, and would suggest a real past situation, not a hypothetical one.
• si habrías traído is not standard Spanish in this structure. After si, Spanish normally does not use the conditional.

So the sentence is following a very common rule:

• Si + hubiera/hubiese + participle
• main clause with conditional

It is the pluperfect subjunctive.

It is made from:

• hubieras = imperfect subjunctive of haber
• traído = past participle of traer

Together:

• hubieras traído = you had brought in a hypothetical/unreal sense

This tense is used for:

• unreal past conditions
• regrets
• hypothetical past situations

In this sentence, it refers to something that did not happen:

• you did not bring more nails

This is a great question, because both are possible in Spanish, but they do not mean exactly the same thing.

estaría terminado

This means:

• it would be finished
• the focus is on the resulting state

So:

• Si hubieras traído más clavos, el estante ya estaría terminado. means something like:
• If you had brought more nails, the shelf would already be finished by now.

This is often called a mixed conditional:

• past unreal condition
• present result

habría estado terminado

This would mean:

• it would have been finished
• the focus is more on a hypothetical completed situation in the past

So the sentence with estaría terminado suggests:

• because you did not bring more nails earlier, the shelf is not finished now
• if you had, it would already be in a finished state now

That is why estaría terminado works very naturally here.

Ya means already here.

It adds the idea that, by this point, the shelf would be finished.

So:

• el estante estaría terminado = the shelf would be finished
• el estante ya estaría terminado = the shelf would already be finished

It often suggests:

• the speaker expected the result by now
• there is a contrast with the current reality

In other words, ya makes the sentence feel more like:

• By now, it would already be done.

Because terminado agrees with el estante, which is masculine singular.

In estar + past participle used as an adjective, the participle agrees in gender and number with the noun:

• el estante está terminado
• la mesa está terminada
• los estantes están terminados
• las mesas están terminadas

Here:

• estante = masculine singular
• so it must be terminado

Because más often works directly with a noun in Spanish, just like more in English.

• más clavos = more nails

You do not need an article here.

Compare:

• Necesito más agua. = I need more water.
• Compró más libros. = He bought more books.

You could add an article in some contexts, but the meaning changes:

• más clavos = more nails in general
• más de los clavos would be a very different structure and not what is meant here

So más clavos is the normal and natural phrasing.

Because the accent mark shows that the vowels are pronounced in separate syllables.

• traído is pronounced roughly like tra-EE-do
• not as a single smooth diphthong

This is a regular pattern in many past participles ending in -aído or -eído:

• traer → traído
• caer → caído
• oír → oído
• leer → leído

The accent helps show the stress and syllable break.

Yes. Both are correct.

Spanish has two common forms of the imperfect and pluperfect subjunctive:

• hubieras traído
• hubieses traído

They mean the same thing here.

In Spain, both forms are used. Some speakers may prefer one over the other depending on style or region, but grammatically both are fine.

So these are both correct:

• Si hubieras traído más clavos...
• Si hubieses traído más clavos...

It is very close, but not exactly the same in this sentence.

A classic English third conditional is:

• If you had brought more nails, the shelf would have been finished.

The Spanish equivalent would be:

• Si hubieras traído más clavos, el estante habría estado terminado.

But your sentence says:

• el estante ya estaría terminado

That is slightly different. It gives a present result of a past unreal condition:

• because you did not bring more nails in the past, the shelf is not finished now
• if you had brought them, it would already be finished now

So this is better understood as a mixed conditional:

• past condition
• present state/result

Yes. Spanish allows that very naturally.

You can say:

• Si hubieras traído más clavos, el estante ya estaría terminado.
• El estante ya estaría terminado si hubieras traído más clavos.

Both mean the same thing.

A small punctuation note:

• when the si clause comes first, a comma is normally used
• when it comes second, the comma is usually omitted

So:

• Si hubieras traído más clavos, el estante ya estaría terminado.
• El estante ya estaría terminado si hubieras traído más clavos.

Yes, el estante usually means the shelf or sometimes the shelving unit, depending on context.

In this sentence, it most naturally means:

• a shelf that someone is building or assembling

A learner should also know these related words:

• el estante = shelf
• la estantería = bookcase, shelving unit, set of shelves

So if the meaning shown to the learner is the shelf, el estante is completely natural.

Yes, often you could, depending on nuance.

• terminado = finished, completed
• acabado = finished, done

In many contexts they overlap:

• El estante ya estaría terminado.
• El estante ya estaría acabado.

However, terminado often sounds especially natural when talking about completing a task or construction process.

Also, in some contexts acabado can have other meanings, so terminado may feel clearer for learners.

In this sentence, terminado is a very good choice.

No. Estar terminado is the natural choice here.

• estar terminado describes the state of being finished
• ser terminado would usually sound odd here

When Spanish talks about the result of a completed action, it often uses:

• estar + past participle

Examples:

• La comida está preparada.
• La puerta está cerrada.
• El estante está terminado.

So:

• el estante ya estaría terminado = the shelf would already be finished

This is about the shelf’s condition, not about an event being passively described with ser.