Prima che gli ospiti si siedano, condisco il cetriolo e la mozzarella con olio e basilico.

Use prima che when it is followed by a full clause with its own subject and a conjugated verb:

prima che gli ospiti si siedano

Use prima di when it is followed by an infinitive, usually when the subject stays the same:

Prima di sedermi, preparo la tavola.

So in this sentence, gli ospiti are the subject of the second action, so prima che is the correct choice.

Because prima che normally requires the subjunctive in Italian.

This is standard grammar: after prima che, the action is seen as something that has not happened yet at the moment of the main action. So Italian uses the subjunctive even if there is no real doubt.

That is why you get:

prima che gli ospiti si siedano

not standard prima che gli ospiti si siedono.

It is part of the reflexive verb sedersi, which means to sit down.

So:

• sedersi = to sit down
• si siedano = that they sit down

The si matches gli ospiti because the guests are doing the action to themselves, in the sense of taking a seat.

Because sedersi is irregular in the present forms.

Its present forms use sied- in many persons:

• mi siedo
• ti siedi
• si siede
• ci sediamo
• vi sedete
• si siedono

And the present subjunctive is:

• che io mi sieda
• che tu ti sieda
• che lui/lei si sieda
• che noi ci sediamo
• che voi vi sediate
• che loro si siedano

So si siedano is the correct present subjunctive form for they sit down.

Because ospiti begins with a vowel sound, and the masculine plural definite article before a vowel is gli.

So:

• l'ospite = the guest
• gli ospiti = the guests

Also, ospite can be masculine or feminine in the singular depending on the person, but gli ospiti usually means a masculine or mixed group. For an all-female group, you would say le ospiti.

The verb is condire, and it belongs to the group of -ire verbs that add -isc- in parts of the present tense.

So the present indicative is:

• io condisco
• tu condisci
• lui/lei condisce
• noi condiamo
• voi condite
• loro condiscono

So condisco is completely normal and is the correct I form.

In this sentence, il cetriolo and la mozzarella are the direct objects, treated as the specific food items being prepared.

After con, Italian often leaves out the article when mentioning ingredients in a general way:

• con olio e basilico
• con sale e pepe
• con zucchero e cannella

If you wanted to stress particular, already-known ingredients, you could say:

con l'olio e il basilico

But in a cooking context, the article is often omitted after con.

Because one con can govern both nouns when they are joined by e.

So:

con olio e basilico

means with oil and basil.

You could repeat the preposition, but it is usually unnecessary:

con olio e con basilico

That version sounds more emphatic or separated, not the most natural neutral phrasing here.

Because they are being used as ingredient names, in a general sense.

In cooking language, Italian often uses the singular for ingredients:

• olio
• basilico
• sale
• pepe

So basilico here means basil as an ingredient, not necessarily one single leaf. If you wanted to focus on individual leaves, you might say foglie di basilico.

Italian often uses the present tense for actions that are about to happen, are planned, or are part of a sequence.

So condisco can naturally mean something like:

• I season
• I'm seasoning
• I season it right before...
• I’ll season it right before...

The present tense is very common for this kind of immediate or routine action.

Yes. You can also say:

Condisco il cetriolo e la mozzarella con olio e basilico prima che gli ospiti si siedano.

That has the same basic meaning.

When the prima che clause comes first, as in your sentence, a comma is very natural:

Prima che gli ospiti si siedano, ...

When the main clause comes first, the comma is often omitted.

In standard Italian, no. After prima che, you should use the subjunctive:

prima che gli ospiti si siedano

Using the indicative si siedono is a very common learner mistake because English does not make this distinction in the same way, but it is not the standard form you should aim for.