Spero che l’antibiotico faccia effetto prima di domani mattina.

Because after spero che in standard Italian, you normally use the subjunctive.

• fa = indicative present
• faccia = subjunctive present

So:

• Spero che l’antibiotico faccia effetto = I hope the antibiotic works / takes effect

Italian often uses the subjunctive after verbs of hoping, doubting, fearing, wanting, and similar attitudes, because the action is not presented as a fact but as something hoped for.

Faccia is the third-person singular present subjunctive of fare.

The relevant forms are:

• che io faccia
• che tu faccia
• che lui/lei faccia
• che noi facciamo
• che voi facciate
• che loro facciano

In this sentence, the subject is l’antibiotico, which is singular, so you need faccia.

Che introduces the second clause: that.

So the structure is:

• Spero = I hope
• che l’antibiotico faccia effetto = that the antibiotic takes effect / works

In English, that is often optional: I hope (that) the antibiotic works.
In Italian, che is normally required in this kind of sentence.

Fare effetto is a very common expression meaning:

• to work
• to take effect
• to have an effect

Literally, it is to make effect, but you should learn it as a fixed Italian expression.

Examples:

• Questo farmaco fa effetto subito. = This medicine works right away.
• L’anestesia non ha ancora fatto effetto. = The anesthesia hasn’t taken effect yet.

So in your sentence, l’antibiotico faccia effetto means the antibiotic works / takes effect.

Because the noun begins with a vowel, and Italian uses l’ before many singular nouns that start with a vowel.

Here, antibiotico is masculine singular, and instead of il antibiotico or lo antibiotico, Italian uses:

• l’antibiotico

This is called elision.

Compare:

• il farmaco
• l’antibiotico
• l’amico
• l’ospedale

Because Italian usually omits subject pronouns when they are already clear from the verb form.

• Spero already means I hope
• so io spero is possible, but usually unnecessary

Italian uses io mainly for emphasis, contrast, or clarity.

For example:

• Spero che guarisca presto. = I hope he/she recovers soon.
• Io spero, ma lui no. = I hope so, but he doesn’t.

Because domani mattina is a time expression, not a full clause.

Use:

• prima di + noun / pronoun / infinitive
• prima che + clause

So here:

• prima di domani mattina = before tomorrow morning

But if you had a full clause, you would use prima che:

• prima che arrivi il medico = before the doctor arrives

And after prima che, Italian normally uses the subjunctive.

Yes. It is a very common time expression meaning tomorrow morning.

In this sentence, with prima di, it means:

• before tomorrow morning
• or more naturally in English, often by tomorrow morning

So the whole sentence suggests the speaker hopes the antibiotic will start working sometime before morning comes.

Yes. Domattina and domani mattina both mean tomorrow morning.

• prima di domani mattina
• prima di domattina

Both are possible.
Domani mattina may sound a little more neutral or explicit, while domattina is very common in everyday speech.

Yes, in this sentence it sounds natural and expected.

Italian uses the definite article more often than English does, especially with concrete nouns referring to a specific thing already understood in context.

So:

• Spero che l’antibiotico faccia effetto = natural
• Spero che antibiotico faccia effetto = incorrect

The article tells us we are talking about the antibiotic the person is taking.

Yes, a little, but the original order is the most natural.

Standard order:

• Spero che l’antibiotico faccia effetto prima di domani mattina.

You could move the time expression for emphasis:

• Spero che prima di domani mattina l’antibiotico faccia effetto.

That version is still correct, but it puts more focus on before tomorrow morning.

For everyday use, the original sentence is the most straightforward choice.