Learn Hardcore Spanish (Latin America): El ascensor se llena rápido cuando termina la clase al mediodía. - The elevator fills up quickly when class ends at noon.

It’s the “pasiva refleja” (reflexive passive) using se to indicate that the elevator fills up by itself (i.e., people enter it) without naming who does it. In English: “The elevator fills up quickly.”

Is se here a reflexive pronoun like in me levanto?

Not exactly. In a true reflexive, the subject and object are the same (I wake myself up). Here, se turns the verb into an impersonal/passive construction, not that the elevator is “waking itself” but that something happens to it.

What is the subject of termina in cuando termina la clase?

The subject is la clase. Spanish can drop subject pronouns, but the verb ending -a already tells you it’s ella (la clase).

Why is rápido used instead of rápidamente?

Spanish often uses adjectives adverbially in everyday speech. Rápido works as “quickly,” while rápidamente is more formal or emphatic.

Why do we say al mediodía and not a mediodía or a las doce?

Al is the contraction of a + el, so al mediodía means “at midday/noon.” You could say a las doce to be precise about 12:00, but al mediodía is a set expression for the middle of the day.

Could the sentence start with the time clause? For example, Cuando termina la clase al mediodía, el ascensor se llena rápido?

Yes. Spanish word order is flexible. Placing the time clause first emphasizes when it happens, but the meaning stays the same.

Is ascensor used everywhere in Latin America?

Ascensor is understood by all Spanish speakers. In some regions (Mexico, parts of Central America) people also say elevador, but both mean “elevator.”

Why does mediodía have an accent on the í?

Because there’s a hiatus between i and a (me-dio-dí-a). The accent marks the stressed syllable and breaks what would otherwise be a diphthong, signaling two separate syllables.