En cuanto llegue el copresentador, ensayaremos la entradilla.

Because en cuanto means as soon as / once, and here it refers to a future event that has not happened yet.

In Spanish, when a time clause like en cuanto, cuando, hasta que, or tan pronto como refers to the future, Spanish usually uses the present subjunctive:

• En cuanto llegue... = As soon as he arrives...

If you say en cuanto llega, that usually sounds like a habitual action or something that regularly happens:

• En cuanto llega, se pone a hablar. = As soon as he arrives, he starts talking.

So in your sentence, llegue is used because the arrival is still pending.

Here, en cuanto means as soon as or once.

It introduces the moment after which the main action will happen:

• En cuanto llegue el copresentador, ensayaremos la entradilla.
• As soon as the co-host arrives, we’ll rehearse the intro.

It is a very common expression in Spanish for talking about time.

Because the sentence is talking about something that will happen later:

• first, the co-host will arrive
• then, we will rehearse the intro

So ensayaremos is the future simple of ensayar.

That said, Spanish often uses the present tense for future meaning in everyday speech too:

• En cuanto llegue el copresentador, ensayamos la entradilla.

This can also sound natural in conversation.
Using ensayaremos simply makes the future meaning very explicit.

Yes. In many contexts, especially in spoken Spanish, you could say:

• En cuanto llegue el copresentador, ensayamos la entradilla.

This still means we’ll rehearse the intro as soon as the co-host arrives.

Spanish often uses the present for near-future or planned actions.
The version with ensayaremos is a bit more formally future in tone, but both are grammatical.

Copresentador means co-host, co-presenter, or co-anchor, depending on context.

It is formed from:

• co- = co-
• presentador = presenter / host

So it refers to someone who presents a programme together with another presenter.

If the person is female, you would usually say:

• la copresentadora

Spanish often uses the definite article where English may or may not use one.

Here, el copresentador means the co-host, referring to a specific person already known in the situation.

Spanish generally requires an article before a singular countable noun in cases like this, whereas English sometimes structures things a bit differently.

Entradilla is a media/broadcasting word. In this context, it usually means the intro, opening lines, or lead-in at the start of a programme or segment.

It comes from entrada and has a diminutive-style ending, but in practice it functions as a set term in journalism, radio, and television.

Depending on context, it can refer to:

• the short introductory speech before a segment
• the opening text or lead
• the intro read by a presenter

So in this sentence, they are going to rehearse the opening introduction.

Not especially in general everyday conversation. It is more common in:

• journalism
• radio
• television
• media production

A learner might hear it less often than more general words like:

• introducción
• intro
• presentación

But in broadcasting, entradilla is perfectly natural.

Because only the first clause is introduced by en cuanto and refers to a future, not-yet-realised action:

• En cuanto llegue el copresentador...

That triggers the subjunctive.

The second clause is the main statement, not a subordinate time clause:

• ensayaremos la entradilla

That part simply states what will happen, so it uses the indicative, here in the future tense.

Yes. You can reverse the clauses:

• Ensayaremos la entradilla en cuanto llegue el copresentador.

This means the same thing: We’ll rehearse the intro as soon as the co-host arrives.

Both orders are natural.
Starting with En cuanto llegue... puts a little more emphasis on the condition in time.

In standard Spain Spanish, llegue is pronounced approximately like YEH-gheh, though the exact sound of ll varies across the Spanish-speaking world.

A few useful points:

• ll is usually pronounced like y in many modern accents
• gue here has a hard g sound, like in get, not like English George
• the u in gue is silent

So:

• llegue ≈ YE-gue / YE-ghe

The exact sound will depend on accent, but the silent u is important.

Both can be possible, but ensayar is the more natural verb for rehearse in performance or media contexts.

• ensayar = to rehearse
• practicar = to practise

So if presenters are preparing lines for a show, ensayar is the most idiomatic choice.

Compare:

• Vamos a ensayar la entradilla. = We’re going to rehearse the intro.
• Voy a practicar mi pronunciación. = I’m going to practise my pronunciation.

No. It depends on whether the action is seen as future/not yet completed or habitual/already factual.

Use the subjunctive when the action is still pending:

• En cuanto llegue, te llamo. = As soon as he arrives, I’ll call you.

Use the indicative for habits or repeated facts:

• En cuanto llega, te llama. = As soon as he arrives, he calls you.

So en cuanto does not automatically require the subjunctive; the time reference is what matters.