Learn Hardcore Spanish (Latin America): El autobús se atrasa cuando llueve.

Se atrasa comes from the pronominal verb atrasarse, which means “to be delayed” or “to run late.” In El autobús se atrasa, it literally means “the bus gets delayed” → “the bus is late.”

Why is se used in se atrasa?

The se makes atrasar into atrasarse, turning a transitive verb (“to delay something”) into an intransitive/pronominal one (“to be delayed”). Without se, atrasar needs an object:

La lluvia atrasó el autobús. (The rain delayed the bus.)
With se, the subject undergoes the action itself:
El autobús se atrasa. (The bus is delayed.)

Why don’t we say El autobús atrasa (without se)?

Without se, atrasa would be transitive and expect a direct object (what it delays). Saying El autobús atrasa would leave that object missing and sound like “the bus delays [something].” To express “the bus is delayed,” Spanish uses the pronominal atrasaRSE.

Could we use se retrasa instead of se atrasa?

Yes. Retrasarse is a synonym of atrasarse in this context. Both mean “to be delayed” or “to run late.” You’ll hear either version in Latin America:

El autobús se retrasa cuando llueve.

Why is the verb llueve in the third-person singular?

Llover is an impersonal verb used only in the third-person singular (similar to “it rains” in English). You cannot say llueven because rain isn’t plural.

Why is cuando followed by the indicative llueve, not the subjunctive?

After cuando (“when”), Spanish uses the indicative if you’re talking about a habitual or factual action:

Cuando llueve, se atrasa. (When it rains, it’s always delayed.)
You’d use the subjunctive only for future or uncertain events:
Cuando llueva, te llamaré. (When it rains [at some future time], I will call you.)

What’s the difference between cuando llueve and si llueve?

Cuando llueve implies a regular, expected condition (“whenever/when it rains”).
Si llueve means “if it rains” and introduces a condition that may or may not happen. Compare:

Cuando llueve, el autobús se atrasa. (Habitual: every time it rains.)
Si llueve mañana, el autobús se atrasará. (Conditional: in case it rains tomorrow.)

Why use el autobús instead of un autobús?

Using the definite article el with a singular noun often expresses a general truth or habitual fact about that entire category:

El perro ladra. (Dogs bark.)
Similarly, El autobús se atrasa cuando llueve can mean “(Any) bus runs late when it rains” or refer to “the usual bus” on a route. Using un autobús would sound like you’re talking about some unspecified single bus, not a general habit.