Si el precinto estuviera roto, llamaría al fabricante.

It is the classic Spanish pattern for a hypothetical or unlikely condition:

si + imperfect subjunctive + conditional

So here:

• Si el precinto estuviera roto = the imagined condition
• llamaría al fabricante = the result

This is very similar to the English If ..., I would ... pattern.

Because Spanish normally uses the imperfect subjunctive after si when the condition is hypothetical, unlikely, or contrary to reality.

So:

• Si el precinto estuviera roto... = If the seal were broken...

You do not use the conditional after si in this kind of sentence.

Because it expresses the consequence of that hypothetical situation.

In this structure:

• the if-clause uses the imperfect subjunctive
• the result clause uses the conditional

So:

• Si el precinto estuviera roto, llamaría...
• If the seal were broken, I would call...

The conditional here means would call, not a real action that is actually happening.

Yes. Estuviera and estuviese are both correct imperfect subjunctive forms.

So these mean the same thing:

• Si el precinto estuviera roto...
• Si el precinto estuviese roto...

In modern Spanish, both are used. The -ra form is often more common in everyday speech.

Because roto here describes a state: the seal is in a broken condition.

Spanish usually uses:

• estar + past participle/adjective for a resulting state

So:

• estar roto = to be broken

Using ser would sound like a passive construction and is not the natural choice here.

A related alternative would be:

• Si el precinto se rompiera, llamaría al fabricante.

That means If the seal broke/were to break, I would call the manufacturer, focusing more on the event of breaking rather than the state of being broken.

In Spain, precinto usually means a seal, especially a security seal, tamper seal, or some official seal that shows whether something has been opened or altered.

It is more specific than a general word like sello. In product or packaging contexts, precinto is a very natural word.

Because roto agrees with precinto, and precinto is:

• masculine
• singular

So the adjective must also be masculine singular:

• el precinto roto

If the noun were feminine, you would change the adjective:

• la tapa rota
• la caja rota

Because Spanish often leaves out subject pronouns when the verb ending already shows who the subject is.

• llamaría already means I would call

So yo is unnecessary unless you want emphasis or contrast:

• Yo llamaría al fabricante, pero tú no.

Without emphasis, leaving the pronoun out sounds more natural.

Because a + el contracts to al.

So:

• a el fabricante → al fabricante

This contraction is required in normal Spanish.

Here, llamar a alguien means to call someone, so the a belongs to the verb.

Usually, no.

Even though estuviera and llamaría look like past forms, this sentence normally refers to a hypothetical present or future situation.

It means something like:

• If the seal were broken, I would call the manufacturer

So it is about an imagined situation now or later, not about something that really happened in the past.

If you wanted a past unreal idea, you would say:

• Si el precinto hubiera estado roto, habría llamado al fabricante.

Yes, but the meaning changes.

• Si el precinto está roto, llamaré al fabricante. = a real, possible, or likely condition
  = If the seal is broken, I will call the manufacturer

• Si el precinto estuviera roto, llamaría al fabricante. = a more hypothetical or less direct condition
  = If the seal were broken, I would call the manufacturer

So the original sentence sounds more tentative or hypothetical.

Not if you want the same meaning.

Si el precinto estaba roto uses the imperfect indicative, not the subjunctive, so it does not create the same hypothetical sense.

That version would tend to sound like background information in the past, or possibly a repeated past situation depending on context.

For the hypothetical If the seal were broken, I would call..., the correct pattern is:

• Si el precinto estuviera roto, llamaría al fabricante.

Yes. You can also say:

• Llamaría al fabricante si el precinto estuviera roto.

The meaning stays the same.

A small punctuation note:

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

So both of these are standard:

• Si el precinto estuviera roto, llamaría al fabricante.
• Llamaría al fabricante si el precinto estuviera roto.