Learn Hardcore Spanish (Spain): El vaso se llena rápido cuando abro el grifo. - The glass fills quickly when I turn on the tap.

In El vaso se llena rápido, the focus is on what happens to the glass, not on who is doing it. The se makes the verb intransitive here and creates a kind of “middle/passive” meaning:

El vaso se llena rápido. = The glass fills up quickly. (It gets full quickly.)

If you said El vaso llena rápido, it would sound like “The glass quickly fills (something else)” – which doesn’t make sense. You need se to show the glass is the thing undergoing the action, not doing it to something else.

Is se here really “reflexive”? Am I saying “the glass fills itself”?

Grammatically it’s the same se form you see in reflexive verbs, but here it’s better understood as a middle / passive use, not truly reflexive.

Reflexive (true): Me lavo. = I wash myself.
Middle / passive-like: El vaso se llena. = The glass gets filled / fills up.

We don’t really imagine the glass “filling itself”, but Spanish uses se this way when:

the agent isn’t important or is obvious (the tap, gravity, you), and
the focus is on the change in state of the subject (the glass becoming full).

Could I say Lleno el vaso rápido cuando abro el grifo instead? What’s the difference?

Yes, you can, but the focus changes:

El vaso se llena rápido cuando abro el grifo.
Focus on the glass and what happens to it: The glass gets full quickly when I open the tap.
Lleno el vaso rápido cuando abro el grifo.
Focus on you as the subject: I fill the glass quickly when I open the tap.

In the first sentence, who does the filling is backgrounded; in the second, you (the speaker) are explicitly doing the action.

Why is it rápido and not rápidamente? Isn’t rápido an adjective?

Rápido is both:

an adjective: un vaso rápido (not natural here, but grammatically: “a fast glass”)
an adverb: Se llena rápido. = It fills up quickly.

In everyday Spanish, it’s very common to use the masculine singular form of many adjectives as adverbs: rápido, lento, claro, fuerte, etc.

Habla rápido. = He/she speaks quickly.
Corre lento. = He/she runs slowly.

Rápidamente is also correct and a bit more formal or careful:

El vaso se llena rápidamente cuando abro el grifo.

Both are fine; rápido is more colloquial and more frequent in speech.

Where does the stress go in rápido and why does it have an accent?

Rápido is pronounced RA-pi-do (stress on the first syllable).

The accent mark is there because:

It is an esdrújula word (stress on the third-from-last syllable),
and in Spanish, all esdrújula words carry a written accent.

So the accent is not optional; it’s required by spelling rules.

Why is it el vaso and not un vaso?

Both are possible, but they suggest slightly different contexts:

El vaso se llena rápido...
Suggests a specific or known glass: maybe “the glass” I’m using, or “the” glass we’ve been talking about.
Un vaso se llena rápido...
Sounds more general or indefinite: “a glass (any glass) fills up quickly...”, maybe as a general statement.

In many real contexts, el vaso is more natural because speakers are usually talking about the particular glass they’re filling right then.

Why is there no yo before abro? Shouldn’t it be yo abro?

Spanish usually omits subject pronouns when the verb ending already makes the subject clear.

(Yo) abro el grifo.
The -o ending in abro already tells you the subject is yo.

You only add yo if you want to emphasize contrast or insist:

Yo abro el grifo, no tú. = I open the tap, not you.

In your sentence, there’s no special emphasis, so cuando abro el grifo is the natural form.

Why is cuando abro el grifo using the present tense and not future or subjunctive?

Here cuando + present indicative is used to state a habitual or general situation:

El vaso se llena rápido cuando abro el grifo.
= Whenever I open the tap, the glass fills up quickly (as a general rule).

Compare:

Talking about a specific future event:
Cuando abra el grifo, el vaso se llenará rápido.
= When I open the tap (on that future occasion), the glass will fill quickly.

So:

Present indicative after cuando → for general, habitual facts.
Subjunctive after cuando → for uncertain, future, or one-off events.

Why is it el grifo? Can I say la llave or something else?

In Spain, el grifo is the standard word for tap / faucet.

In much of Latin America, people more often say:

la llave or la llave del agua = the tap

So in Peninsular Spanish:

Abro el grifo. = I turn on / open the tap.

In Latin American Spanish you’re more likely to hear:

Abro la llave.

Your sentence is clearly in European (Spain) Spanish because of el grifo.

Could the word order be El vaso rápido se llena cuando abro el grifo?

No, that word order is not natural.

The usual positions for rápido in this sentence are:

El vaso se llena rápido cuando abro el grifo.
El vaso se llena cuando abro el grifo, rápido. (more marked/emphatic, like an afterthought)

But putting rápido between el vaso and se llena sounds wrong; Spanish normally keeps the clitic se right before the verb and places adverbs like rápido after the verb (or at the end of the clause) in a neutral sentence.

What is the difference between llenar and llenarse?

Llenar (without se) is transitive: someone/something fills something else.
- Lleno el vaso. = I fill the glass.
- La lluvia llena el río. = The rain fills the river.
Llenarse (with se) is used when the subject is the thing that becomes full:
- El vaso se llena. = The glass gets full / fills up.
- El estadio se llenó. = The stadium filled up.

In your sentence, you’re talking about the glass becoming full, so se llena is the natural choice.

How does llenarse compare to using estar lleno?

They focus on different things:

El vaso se llena rápido.
Focus on the process of becoming full: it fills up quickly.
El vaso está lleno.
Focus on the resulting state: the glass is (now) full.

You could combine them in a little sequence:

El vaso se llena rápido y enseguida está lleno.
= The glass fills up quickly and is full right away.

Could I add agua and say El vaso se llena de agua rápido cuando abro el grifo? Is de correct here?

Yes, that’s correct and natural:

El vaso se llena de agua rápido cuando abro el grifo.

Llenarse de + noun is the normal pattern to say what something gets filled with:

La calle se llenó de gente. = The street filled with people.
La habitación se llenó de humo. = The room filled with smoke.

You could also say con agua in some contexts (se llena con agua), but llenarse de agua is more idiomatic here.