Dehors, la chienne se met à aboyer chaque fois qu’une voiture passe.

Dehors means outside.

At the start of the sentence, it sets the scene: Outside, the dog starts barking every time a car passes. French often puts this kind of adverb first for context.

You could also say the sentence without it if the location is already clear, but adding Dehors makes the image more vivid.

The comma after Dehors is natural because it separates this scene-setting word from the rest of the sentence.

La chienne means the female dog, while le chien means the male dog or sometimes just the dog in a general sense.

So this sentence specifically refers to a female dog.

A learner might wonder whether French really cares about the dog’s sex here. Grammatically, yes: chienne is a different noun, not just a different article.

• le chien = male dog / dog in general
• la chienne = female dog

Se mettre à + infinitive means to start doing something or to begin to do something.

So:

• la chienne se met à aboyer = the dog starts barking

This is a very common French structure.

Some similar expressions are:

• commencer à + infinitive = to begin to
• se mettre à + infinitive = to start to, often with a slightly more immediate or vivid feeling

In everyday French, se mettre à is very natural when someone suddenly begins an action.

Because after se mettre à, French uses an infinitive.

So the structure is:

• se mettre à + infinitive

Examples:

• se mettre à rire = to start laughing
• se mettre à pleurer = to start crying
• se mettre à aboyer = to start barking

Aboie would be a conjugated form, as in:

• la chienne aboie = the dog is barking / barks

But here the main verb is se met, so the next verb must stay in the infinitive: aboyer.

Chaque fois que means every time that or more naturally in English, just every time.

So:

• chaque fois qu’une voiture passe = every time a car passes

It introduces a repeated action. This is why the whole sentence describes a habit:

• whenever a car goes by, the dog starts barking

Other similar expressions are:

• quand = when
• à chaque fois que = every time that
• dès que = as soon as

Compared with quand, chaque fois que makes the repeated nature especially clear.

This is because of elision.

When que comes before a word beginning with a vowel sound, the e drops:

• que une → qu’une

This happens very often in French:

• que il → qu’il
• que elle → qu’elle
• que on → qu’on
• que une → qu’une

So chaque fois qu’une voiture passe is just the normal elided form.

Here passe is present indicative, not subjunctive.

The clause chaque fois qu’une voiture passe states something habitual or regular: every time a car passes.

French uses the present tense here just as English often does in similar time clauses:

• Every time a car passes, the dog barks.

The subjunctive is not needed because this is not expressing doubt, emotion, possibility, or a required reaction. It is simply describing a repeated fact.

So:

• une voiture passe = a car passes / goes by

Yes, in this sentence passer means to pass by or to go past.

French often uses passer by itself in this way, without needing a separate word equivalent to English by.

So:

• une voiture passe = a car passes / a car goes by

Depending on context, passer can also mean other things, such as:

• to spend time: passer la journée
• to take an exam: passer un examen
• to hand something: passer le sel

But here, with une voiture, the meaning is clearly movement past a place.

Because the subject is singular: la chienne.

So the verb must also be singular:

• la chienne se met = the female dog starts
• les chiennes se mettent = the female dogs start

This is just normal subject-verb agreement in French.

You can break it down like this:

• subject: la chienne
• verb: se met
• infinitive phrase: à aboyer

Yes, absolutely.

• la chienne se met à aboyer
• la chienne commence à aboyer

Both mean the dog starts barking.

The difference is mostly one of style and nuance:

• commencer à is very neutral
• se mettre à often feels a bit more dynamic or sudden, as if the action begins abruptly

In a sentence about a dog reacting to passing cars, se met à aboyer sounds especially lively and natural.

It most naturally describes a habitual action.

That comes mainly from:

• chaque fois que = every time
• the present tense se met / passe

So the sense is:

• this is what the dog does whenever a car goes by

French present tense often covers this kind of repeated behavior, just like English simple present:

• The dog barks every time a car passes.

If you wanted one specific moment in the past, French would use different tenses.