Credo che il certificato sia già pronto da ieri.

Because credere che normally triggers the subjunctive in Italian.

• è = indicative present of essere
• sia = subjunctive present of essere

After expressions of opinion, doubt, possibility, emotion, etc., Italian often uses the subjunctive in the subordinate clause:

• Credo che sia vero. = I think it is true.
• Penso che abbia ragione. = I think he/she is right.

So in Credo che il certificato sia già pronto da ieri, the speaker is giving a belief or assumption, not stating a fact with full certainty.

In very informal speech, some native speakers may sometimes use the indicative where the subjunctive is expected, but sia is the standard and correct choice here.

Sia is the present subjunctive of essere.

Here is the present subjunctive of essere:

• che io sia
• che tu sia
• che lui/lei sia
• che noi siamo
• che voi siate
• che loro siano

In this sentence, sia matches il certificato, which is third-person singular.

Here da ieri means since yesterday.

With time expressions, da often marks the starting point of an action or state that began in the past and continues into the present.

So:

• pronto da ieri = ready since yesterday

This is a very common Italian pattern:

• Sono qui da lunedì. = I’ve been here since Monday.
• Piove da stamattina. = It has been raining since this morning.

So the idea is that the certificate became ready yesterday and is still ready now.

Già means already.

In sia già pronto, it modifies the idea of being ready:

• sia pronto = is ready
• sia già pronto = is already ready

Its position is natural and very common in Italian: it comes before the adjective or participle it closely relates to.

You may also hear slightly different word orders for emphasis, but sia già pronto is the most neutral and natural here.

For things like documents, forms, certificates, food, and objects, pronto is a very common Italian way to say ready.

So:

• Il certificato è pronto. = The certificate is ready.

This suggests the certificate has been completed and is available.

Preparato can mean prepared, but it is not always the most idiomatic choice for a document in this kind of context. Pronto sounds more natural if you mean the certificate is finished and ready for use or pickup.

Because pronto agrees with il certificato.

• certificato is masculine singular
• so the adjective must also be masculine singular: pronto

Agreement patterns:

• il certificato pronto = masculine singular
• la lettera pronta = feminine singular
• i certificati pronti = masculine plural
• le lettere pronte = feminine plural

Italian uses articles more often than English does.

Here il certificato refers to a specific certificate, probably one already known from the context. So the definite article is natural and expected.

Compare:

• Il certificato è pronto. = The certificate is ready.
• Un certificato would mean a certificate, introducing it less specifically.

In many contexts where English is more flexible, Italian still prefers an article.

No, not in standard Italian.

In English, you can say:

• I think that the certificate is ready
• I think the certificate is ready

But in Italian, after credo, you normally need che before a full clause:

• Credo che il certificato sia pronto.

Without che, the sentence would sound wrong.

Yes, it is natural.

Credo che il certificato sia già pronto da ieri is a normal sentence and means that the speaker thinks the certificate has been ready since yesterday.

A few close alternatives are also possible, depending on emphasis:

• Credo che il certificato sia pronto da ieri.
• Penso che il certificato sia già pronto da ieri.
• Credo che il certificato sia pronto già da ieri.

These are all similar, though the exact emphasis shifts slightly.

If you wanted to say I think the certificate was already ready yesterday, that would be a different idea, and you might say:

• Credo che ieri il certificato fosse già pronto.

So the original sentence suggests a state that started yesterday and continues now.

A rough breakdown is:

• Credo = I believe / I think
• che = that
• il certificato = the certificate
• sia = be
• già = already
• pronto = ready
• da ieri = since yesterday

So the structure is very close to English:

• I think that the certificate be already ready since yesterday

That literal English is not natural, of course, but it helps show how the Italian sentence is built. The real English equivalent would be something like:

• I think the certificate has been ready since yesterday.