Quand la musique commence, les enfants se mettent à danser.

Quand means when here. It introduces a time clause: Quand la musique commence.

In this sentence, that clause sets the scene first, and then the main action comes after it:

• Quand la musique commence, ...
• les enfants se mettent à danser.

French often works this way, just like English: When the music starts, the children begin to dance.

Because the sentence starts with a time clause introduced by quand. French commonly uses a comma to separate that opening clause from the main clause:

• Quand la musique commence, les enfants se mettent à danser.

You can also reverse the order:

• Les enfants se mettent à danser quand la musique commence.

In that version, the comma is usually not needed.

French uses articles much more often than English does. So where English may say music or children, French often needs the-type words:

• la musique = the music
• les enfants = the children

Even when English sounds more natural without an article, French may still require one.

Because commence is the conjugated verb form, while commencer is the infinitive.

The subject is la musique, which is third-person singular, so commencer becomes commence in the present tense:

• je commence
• tu commences
• il / elle / on commence
• nous commençons
• vous commencez
• ils / elles commencent

Here, la musique is singular, so French uses commence.

That comes from the verb expression se mettre à + infinitive, which means to begin to, to start to, or sometimes to set about doing something.

The base form is:

• se mettre à danser = to begin to dance

Since the subject is les enfants, the verb is conjugated in the plural:

• les enfants se mettent à danser

So se mettent is the present tense form for they begin.

Se is the reflexive pronoun that belongs to the verb se mettre à.

This expression is normally used as a pronominal/reflexive verb:

• je me mets à
• tu te mets à
• il se met à
• nous nous mettons à
• vous vous mettez à
• ils se mettent à

So in les enfants se mettent à danser, the se matches les enfants.

You should learn se mettre à as a whole expression, not just mettre by itself.
Mettre alone usually means to put, but se mettre à means to start doing something.

Because the expression is se mettre à + infinitive.

So the pattern is:

• se mettre à chanter = to start singing
• se mettre à rire = to start laughing
• se mettre à courir = to start running
• se mettre à danser = to start dancing

The à is required by this construction.

Because se mettre à danser focuses on the beginning of the action.

Compare:

• les enfants dansent = the children are dancing / dance
• les enfants se mettent à danser = the children begin to dance / start dancing

So the sentence is not just saying that they dance. It is emphasizing the moment when the dancing starts.

French often uses the present tense for general truths, repeated situations, vivid narration, or instructions about what normally happens.

So this sentence can mean something like:

• Whenever the music starts, the children begin to dance or
• When the music starts, the children start dancing

It is presenting the action as a typical or immediate sequence, not necessarily tied to one specific past event.

It can suggest either, depending on context.

In a sentence like this, with the present tense, quand often has a general sense close to whenever:

• Quand la musique commence, les enfants se mettent à danser.
• Whenever the music starts, the children start dancing.

But in some contexts, English may still naturally translate it as when. The broader situation tells you which nuance fits best.

Because French spelling shows grammatical information even when the ending is silent.

With les enfants, the verb must be third-person plural, so mettre becomes mettent.

Even though -ent is usually silent in spoken French, it still matters in writing because it shows the subject is plural.

Yes. In careful and standard pronunciation, there is usually a liaison between les and enfants.

So les enfants is pronounced roughly like:

• lay-zahn-fahn

The s in les is normally silent, but before a vowel sound it links and sounds like z.

Yes. You can also say:

• Les enfants se mettent à danser quand la musique commence.

That means the same thing. The difference is mostly one of emphasis and rhythm:

• Quand la musique commence, ... puts the time setting first.
• Les enfants se mettent à danser ... puts the children’s action first.

Both are natural French.

Yes. Grammar follows the noun, not the real-world idea behind it.

La musique is a singular noun in French, so the verb is singular too:

• la musique commence

The same thing happens in English with the music starts.

Yes, often you can:

• Les enfants commencent à danser.

That also means The children begin to dance.

But se mettre à + infinitive is very common and often feels a bit more dynamic, as if the action suddenly gets underway. In this sentence, it gives a lively sense of the children launching into dancing as soon as the music begins.