Cuando contó el chiste, toda la mesa se echó a reír a carcajadas.

Because contó is the pretérito form, which presents the action as a completed event: he/she told the joke.

In this sentence, the joke-telling is a specific finished action that triggers what happened next. That is why contó fits well.

If you said cuando contaba el chiste, it would sound more like while he/she was telling the joke or whenever he/she told the joke, depending on context. That would shift the meaning.

So:

• cuando contó el chiste = when he/she told the joke
• cuando contaba el chiste = while he/she was telling the joke / when he/she used to tell the joke

The subject is not stated explicitly, but Spanish often leaves it out because the verb form already gives the person and number.

Contó means he/she/you formal told.

So the sentence could refer to:

• he told the joke
• she told the joke
• you told the joke (formal usted)

The exact subject would come from the wider context.

In Spanish, it is very common to use the definite article where English might not.

So contó el chiste is a natural way to say told the joke.

It usually refers to a particular joke in the situation, even if English might also simply say told a joke in some contexts. Spanish often prefers the article with direct objects when the thing is understood or identifiable in context.

Literally, toda la mesa means the whole table.

But here mesa does not refer just to the piece of furniture. It refers to all the people sitting at the table. This is a very common kind of expression in both Spanish and English.

It is similar to saying in English:

• The whole table laughed
• Table 6 is waiting for the bill

So this is a kind of shorthand: the table stands for the group gathered around it.

Because mesa is a feminine singular noun.

The word todo changes to agree with the noun:

• todo el libro
• toda la mesa
• todos los chicos
• todas las chicas

Since mesa is feminine singular, the correct form is toda.

Echarse a + infinitive is an expression meaning to begin suddenly doing something.

So se echó a reír means:

• he/she burst out laughing
• he/she started laughing
• he/she suddenly began to laugh

In this sentence, because the subject is toda la mesa, it means the whole table burst out laughing.

This structure often gives a sense of a quick, sudden start.

Because the expression is idiomatic: echarse a + infinitive.

The se is part of the fixed verbal expression, not a separate reflexive meaning like himself/herself in a literal sense.

Compare:

• echó el libro a la mesa = he threw the book onto the table
• se echó a reír = he burst out laughing

In the second one, echarse a reír functions as a set phrase meaning to start laughing suddenly.

Because after a in the expression echarse a + infinitive, Spanish uses the infinitive.

So:

• se echó a reír = started to laugh / burst out laughing

Not:

• se echó a rió ❌

That would be ungrammatical, because rió is a finite verb form, and this structure requires an infinitive.

A carcajadas means laughing loudly, uproariously, or in great bursts of laughter.

A carcajada is a loud, hearty laugh.

So:

• reír a carcajadas = to laugh out loud / to laugh uproariously

It adds intensity. Without it, se echó a reír just tells you that they started laughing. With a carcajadas, you understand it was strong, noisy laughter.

Because they belong to two different structures:

• echarse a reír = to start laughing
• reír a carcajadas = to laugh loudly / uproariously

So the first a is part of echarse a + infinitive, and the second a is part of the expression a carcajadas.

It may look repetitive, but it is completely natural.

Yes, usually.

• rió simply means laughed
• se echó a reír means started laughing or burst out laughing

So se echó a reír focuses on the moment the laughter began, often with a more vivid or sudden feel.

Adding a carcajadas makes it even stronger:

• rió = laughed
• se echó a reír = burst out laughing
• se echó a reír a carcajadas = burst out laughing loudly / in fits of loud laughter

Yes, but the nuance changes slightly.

• se puso a reír = started laughing
  This is common and fairly neutral.

• se echó a reír = burst out laughing
  This often sounds a bit more vivid or sudden.

• rompió a reír / romper a reír = burst into laughter
  This exists, but it is more literary or less common in everyday speech.

So the original sentence sounds very natural and expressive.

Because the sentence begins with a subordinate clause introduced by cuando:

• Cuando contó el chiste, ...

In Spanish, when this kind of time clause comes first, it is normal to separate it from the main clause with a comma.

If the order were reversed, the comma is often omitted:

• Toda la mesa se echó a reír a carcajadas cuando contó el chiste.

Both are correct; the original simply places the when clause first.

Here it means when, referring to one specific occasion.

That is clear because contó is in the preterite, which points to a completed event.

If you wanted whenever, Spanish would often use a different tense pattern, for example:

• Cuando contaba ese chiste, toda la mesa se reía.

That gives a repeated or habitual sense: Whenever he/she told that joke, the whole table laughed.

Because echó is the third-person singular preterite of echar:

• yo eché
• él/ella/usted echó

The accent mark is important because it distinguishes this past-tense form from echo, which is a different word:

• echo = I throw or I cast from the verb echar, in the present tense
• echo can also be a noun meaning fact, from hecho, in speech they sound similar in many accents

So in this sentence, echó must have the accent.

Yes, it sounds very natural in Spain Spanish.

All of these elements are normal and idiomatic:

• contó un/el chiste
• toda la mesa
• se echó a reír
• a carcajadas

None of them sounds strange or overly formal. The whole sentence is a vivid, natural way to describe a group suddenly laughing hard after someone told a joke.

Normally, no. The verb agrees with the grammatical subject, and the grammatical subject here is la mesa, which is singular.

So:

• Toda la mesa se echó a reír ✅

Not:

• Toda la mesa se echaron a reír ❌

Even though the meaning refers to multiple people, the noun mesa is singular, so the verb stays singular too. This is similar to how collective nouns work in many languages.