Mi hermana se echó a reír al escuchar el chiste gracioso.

Se echó a reír is an idiomatic expression meaning she burst out laughing or she started laughing.

Literally, echarse a + infinitive can mean something like to throw oneself into doing something, but in real Spanish this structure is often just used to show the start of an action.

So:

• se echó a reír = she started to laugh / she burst out laughing
• A simpler version would be rió = she laughed

The version with echarse a reír sounds more expressive because it emphasizes the moment laughter began.

The se is part of the fixed expression echarse a + infinitive.

Here, echar by itself means to throw, to pour, to cast, and other things depending on context. But echarse a is a different construction, often used to mean to begin doing something suddenly.

So you should learn these as a unit:

• echarse a llorar = to start crying
• echarse a correr = to start running
• echarse a reír = to start laughing

In this sentence, se does not mean herself in the usual literal sense. It is simply part of the verb phrase.

Echó is the preterite form of echarse for él/ella/usted.

The sentence describes a completed event in the past: your sister heard the joke and then burst out laughing. That is exactly the kind of situation where Spanish often uses the preterite.

• se echó a reír = she burst out laughing
• se echaba a reír would suggest a repeated or habitual action, or background description, depending on context

So echó is used because this is a specific past event.

The accent mark distinguishes echó from echo.

• echó = he/she/it threw, or here, she started
• echo = I throw or echo as a noun in English-derived contexts

In this sentence, you need echó because the subject is mi hermana, so the verb must be in the third person singular preterite.

Because the pattern is echarse a + infinitive.

After a, Spanish uses the infinitive:

• echarse a reír
• echarse a llorar
• echarse a correr

So reír stays in the infinitive form because it is part of this structure.

Both relate to laughing, but they are not always used in exactly the same way.

• reír = to laugh
• reírse = to laugh, often very commonly used in everyday speech

In many contexts, reír and reírse are very close in meaning. But in this sentence, the expression is specifically echarse a reír, not echarse a reírse.

So even though you may often see mi hermana se rió, with this idiom you say:

• se echó a reír

not

• se echó a reírse

Al escuchar means upon hearing, when hearing, or on hearing.

It is a very common Spanish structure:

• al + infinitive

This often expresses that one action happens when or as soon as another happens.

So:

• al escuchar el chiste = when she heard the joke / upon hearing the joke

It connects the two actions smoothly:

1. she heard the joke
2. she burst out laughing

Both are possible, but they are slightly different in style.

• al escuchar el chiste = upon hearing the joke / when hearing the joke
• cuando escuchó el chiste = when she heard the joke

Al + infinitive is a compact way to express that one action happened at the moment another one happened. It can sound a bit more elegant or concise.

If you changed the sentence to:

• Mi hermana se echó a reír cuando escuchó el chiste gracioso

it would still be correct and natural.

It is a contraction of a + el.

So:

• a + el = al

In this sentence:

• al escuchar

comes from a el escuchar historically, but modern Spanish always uses al, not a el, before a masculine singular article.

However, with al + infinitive, it is best to think of the whole pattern as a fixed structure meaning when/upon doing something.

Because after al in this construction, Spanish uses the infinitive.

So:

• al escuchar = upon hearing
• al llegar = upon arriving
• al entrar = upon entering

You do not conjugate the verb there. If you want a conjugated verb, you would usually use a different structure, such as:

• cuando escuchó el chiste

That is why escuchar stays in the infinitive form.

It is understandable, but it can sound a little redundant, because a chiste is already expected to be funny.

• chiste = joke
• gracioso = funny / amusing

So chiste gracioso is a bit like saying a funny joke in English. That is not wrong, but it adds emphasis rather than new information.

A native speaker might simply say:

• Mi hermana se echó a reír al escuchar el chiste.

But chiste gracioso can still work if you want to stress that the joke really was funny.

Usually, yes, gracioso means funny, amusing, or witty.

But it can also have other shades depending on context:

• un chiste gracioso = a funny joke
• un niño muy gracioso = a very funny/amusing child
• sometimes it can mean cheeky or fresh depending on tone and context

In this sentence, it clearly means funny.

In Spain, gracioso is very common for funny, though depending on the context you might also hear divertido. For a joke, gracioso sounds very natural.

Grammatically, yes, but it is less natural in everyday Spanish than se rió or se echó a reír.

More natural alternatives would be:

• Mi hermana se rió al escuchar el chiste. = My sister laughed when she heard the joke.
• Mi hermana se echó a reír al escuchar el chiste. = My sister burst out laughing when she heard the joke.

The version with se echó a reír is more vivid and emphasizes the sudden start of the laughter.

Yes, Spanish word order is somewhat flexible, though some versions sound more natural than others.

Original:

• Mi hermana se echó a reír al escuchar el chiste gracioso.

Possible variation:

• Al escuchar el chiste gracioso, mi hermana se echó a reír.

This version is also perfectly natural and maybe even slightly more elegant in writing.

A less natural version would be:

• Mi hermana se echó a reír al escuchar el gracioso chiste.

This is not wrong, but el chiste gracioso sounds more normal than el gracioso chiste in everyday speech.

Yes. You could say:

• Mi hermana se echó a reír al oír el chiste.

That is also correct.

Very roughly:

• oír = to hear
• escuchar = to listen / to hear attentively

But in many everyday contexts, both can work. In this sentence, al oír el chiste and al escuchar el chiste are both natural, though oír may sound a bit more directly tied to simply hearing the joke.