Le chaton s’éloigne du chiot dès qu’il se met à aboyer.

Because s’éloigner is a reflexive verb, and here it means to move away or to go away.

• éloigner by itself usually means to move something away
  
  • Il éloigne la chaise. = He moves the chair away.
• s’éloigner means to move away oneself
  • Le chaton s’éloigne. = The kitten moves away.

The s’ is the reflexive pronoun se, which becomes s’ before a vowel.

Because the verb s’éloigner is normally followed by de when you say what someone is moving away from.

• s’éloigner de quelque chose / de quelqu’un = to move away from something / someone

So:

• de + le = du

That is why de le chiot becomes du chiot.

Dès que means as soon as.

It introduces the moment when something happens:

• Le chaton s’éloigne du chiot dès qu’il se met à aboyer.
• The kitten moves away from the puppy as soon as it starts barking.

It is a very common expression in French for immediate sequence.

Se mettre à + infinitive is a very common French structure meaning to start doing something.

So:

• se mettre à aboyer = to start barking

Other examples:

• Elle se met à rire. = She starts laughing.
• Ils se mettent à courir. = They start running.

Here, se met à is conjugated, and aboyer stays in the infinitive.

Because the expression is se mettre à + infinitive.

You cannot normally say:

• se mettre aboyer ❌

You need:

• se mettre à aboyer ✅

The à belongs to the whole expression, just like in English we say to start to bark or to begin to bark.

In this sentence, il most naturally refers to le chiot.

Why?

• le chiot is masculine singular
• il is masculine singular
• semantically, it makes sense that the puppy starts barking

So the most natural reading is:

• As soon as the puppy starts barking, the kitten moves away.

Technically, French pronouns can sometimes create ambiguity, but in this sentence context makes the intended meaning clear.

French often uses the present tense for things that are generally true, habitual, or described vividly.

Here, the sentence can mean something like a general situation:

• The kitten moves away from the puppy as soon as it starts barking.

Both actions are presented as happening in a normal, repeated, or immediate sequence, so the present works naturally in French.

In French, object and reflexive pronouns usually go before the conjugated verb.

So:

• se + éloigne → s’éloigne
• se + met → se met

This is different from English, where pronouns are not attached in the same way.

For reflexive verbs, this is the normal pattern:

• je me lève
• tu te dépêches
• il se met à courir

This happens because of elision.

When se comes before a word starting with a vowel or silent h, the e drops:

• se éloigne ❌
• s’éloigne ✅

This is the same kind of change you see in:

• je aime ❌ → j’aime ✅
• le ami ❌ → l’ami ✅

Aboyer is an infinitive verb meaning to bark.

In this sentence, it comes after se mettre à, so it stays in the infinitive:

• il se met à aboyer

That is the normal pattern in French after many verb expressions:

• commencer à parler
• essayer de comprendre
• se mettre à courir

Yes, both chaton and chiot are masculine nouns, so they take le in the singular.

• le chaton = the kitten
• le chiot = the puppy

This is grammatical gender, so it does not necessarily tell you whether the actual animal is male. If you want to specify a female animal, French often uses a different word or adds extra information depending on the animal and context.

Yes. Dès que is very commonly followed by the indicative, including the present when the sentence is talking about a present, habitual, or general situation.

So this is completely normal:

• dès qu’il se met à aboyer

If the whole sentence were in the future, French would often still use the present after dès que:

• Le chaton s’éloignera dès qu’il se mettra à aboyer.

So the tense after dès que depends on the time frame of the sentence, but the indicative is the normal mood here.