Il bambino si mette a ridere quando vede il cane.

Si mette a ridere means starts laughing or bursts out laughing. It focuses on the moment the laughter begins.

Compare:

• Il bambino ride = The child laughs / is laughing
• Il bambino si mette a ridere = The child starts laughing

So this sentence is not just describing laughter; it is showing the reaction happening at that moment.

Here si is part of the verb mettersi.

On its own, mettere means to put. But mettersi a + infinitive is a common expression meaning to begin doing something.

So:

• mettere = to put
• mettersi a ridere = to start laughing
• mettersi a studiare = to start studying
• mettersi a correre = to start running

In this sentence, si does not really mean himself in a strong literal way. It is just part of this idiomatic structure.

After mettersi, Italian normally uses a + infinitive to mean start to do something.

So the pattern is:

• mettersi a + infinitive

Examples:

• si mette a ridere = starts laughing
• mi metto a leggere = I start reading
• si mettono a parlare = they start talking

You cannot normally replace a with another preposition here.

Yes, you could, but the meaning changes slightly.

• Il bambino ride quando vede il cane = The child laughs when he sees the dog
• Il bambino si mette a ridere quando vede il cane = The child starts laughing when he sees the dog

The first version is more neutral. The second emphasizes the beginning of the laughter, often with a sense of immediate reaction.

Italian often leaves out subject pronouns when the subject is already clear from the context or the verb form.

So quando vede il cane naturally means when he sees the dog, with he understood as the child.

Italian does this all the time:

• Mangio = I eat / I am eating
• Parliamo = we speak / we are speaking
• Vede il cane = he/she sees the dog

You could say quando lui vede il cane, but that would usually add emphasis or contrast.

The subject is still il bambino.

So the sentence means:

• The child starts laughing when he sees the dog

By default, Italian usually keeps the same subject unless something else is clearly stated.

If you wanted the dog to be the one seeing, you would need to make that explicit, for example:

• Il bambino si mette a ridere quando il cane lo vede = The child starts laughing when the dog sees him

They are in the present tense because Italian often uses the present for:

• general statements
• habitual actions
• examples
• vivid narration

So this sentence can mean something like:

• The child starts laughing when he sees the dog
• Whenever the child sees the dog, he starts laughing

If you wanted a completed past event, you would usually say:

• Il bambino si è messo a ridere quando ha visto il cane = The child started laughing when he saw the dog

Italian uses articles more often than English does.

• il bambino = the child / the boy
• il cane = the dog

In this sentence, il cane probably refers to a specific dog, or at least a dog that is understood in the context.

If you wanted to mean a dog, you would usually say:

• quando vede un cane = when he sees a dog

So the article choice changes the feeling of specificity.

Yes. You can also say:

• Quando vede il cane, il bambino si mette a ridere.

This means the same thing. The difference is mostly one of style or emphasis.

• Putting quando vede il cane first highlights the time or trigger
• Putting Il bambino first highlights the child

Both are natural.

Si mette is the third-person singular present of mettersi.

The full present tense is:

• mi metto = I start / I put myself
• ti metti = you start
• si mette = he/she starts
• ci mettiamo = we start
• vi mettete = you all start
• si mettono = they start

In this sentence, si mette matches il bambino, which is third-person singular.

Not only for sudden actions, but it often gives the idea of beginning an action, sometimes quite abruptly.

Examples:

• Si mette a piangere = He/She starts crying
• Mi metto a lavorare = I start working
• Si sono messi a cantare = They started singing

With ridere, it often sounds a bit lively or immediate, so si mette a ridere can feel very natural for bursts out laughing.