Learn Spanish (Latin America): Apenas ella llegue, va a calentar la sopa en el microondas. - As soon as she arrives, she is going to heat the soup in the microwave.

Because apenas (as soon as) sets up a time clause referring to a future event. At the moment of speaking, she hasn't arrived yet, so her arrival is anticipated but not yet a reality. This kind of pending future action always triggers the subjunctive.

Wait, doesn't apenas usually mean "barely" or "hardly"?

Yes, in other contexts it does, such as in Apenas tengo tiempo (I barely have time). However, when it is used as a time connector introducing a clause, it translates to "as soon as".

Could I use en cuanto or tan pronto como instead of apenas in this sentence?

Absolutely. All three mean "as soon as" and are completely interchangeable here. Apenas is very common in everyday Latin American Spanish because it is short and natural to say.

Do we really need the subject pronoun ella before llegue?

It isn't strictly required, but it is often used for clarity. The present subjunctive form llegue can mean "he arrives", "she arrives", or "you (formal) arrive". Including ella removes any ambiguity about who is walking through the door.

Why does end in an if it refers to a single microwave?