Après la promenade, le chiot s’est endormi comme s’il avait couru pendant des heures.

Après la promenade means After the walk or After the outing.

• après = after
• la promenade = the walk / the outing

In French, it is very common to place a time expression at the beginning of the sentence to set the scene.

So:

• Après la promenade, le chiot s’est endormi... = After the walk, the puppy fell asleep...

Also, promenade is a feminine noun, which is why it takes la.

Le chiot means the puppy.

French has a specific word for a young dog:

• chien = dog
• chiot = puppy

So le chiot is more precise than le chien. It tells you the animal is young, not just any dog.

Because the verb here is s’endormir, which means to fall asleep.

This is a pronominal verb:

• s’endormir = to fall asleep
• literally, it includes the reflexive pronoun se

In the passé composé, pronominal verbs use être, not avoir:

• il s’est endormi = he fell asleep

Compare:

• il s’est endormi = he fell asleep
• il a endormi le bébé = he put the baby to sleep

So endormir and s’endormir are not the same thing.

Those apostrophes are caused by elision.

French often drops the final vowel of a short word when the next word begins with a vowel or silent h.

So:

• se est becomes s’est
• si il becomes s’il

This is completely standard in French and happens very often:

• je ai → j’ai
• le arbre → l’arbre
• ne a → n’a

So s’est and s’il are just the normal shortened forms.

S’est endormi means fell asleep or went to sleep.

The infinitive is s’endormir.

In this sentence:

• le chiot s’est endormi = the puppy fell asleep

The past participle is endormi.

Because the subject is le chiot (masculine singular), the participle appears as endormi.

If the subject were feminine, you would see:

• la petite chienne s’est endormie

And in the plural:

• les chiots se sont endormis

Comme si means as if.

It introduces a comparison between reality and appearance:

• le chiot s’est endormi comme s’il avait couru pendant des heures = the puppy fell asleep as if he had run for hours

The idea is not that we know for sure he ran for hours. It is how exhausted he seemed.

So comme si is used when something appears to be true, or when you are imagining a situation.

This is one of the most important grammar points in the sentence.

After comme si, French often uses a past tense that shows something imagined, unreal, or only apparent.

Here we get:

• comme s’il avait couru
• literally: as if he had run

avait couru is the plus-que-parfait:

• avait = imperfect of avoir
• couru = past participle of courir

Why the pluperfect here?

Because the sentence presents the running as something that would have happened before the puppy fell asleep, but only in an imagined sense. He looked so tired that it seemed as if he had run for hours.

This is very natural French after comme si.

Normally, no. Comme si usually does not take the straightforward tense you might expect from English.

French tends to use:

• imparfait after comme si for a present/unreal appearance
• plus-que-parfait after comme si for a past/unreal appearance

So:

• Il parle comme s’il savait tout. = He talks as if he knew everything.
• Il était couvert de boue comme s’il avait travaillé dans le jardin. = He was covered in mud as if he had worked in the garden.

In your sentence, the puppy appears to have already done something exhausting before falling asleep, so avait couru fits naturally.

Because couru is the past participle of courir, and here it is used with avoir:

• il avait couru

With avoir, the past participle usually does not agree with the subject.

So even though il is masculine singular, the participle stays:

• il avait couru
• elle avait couru
• ils avaient couru
• elles avaient couru

It would only agree in certain cases with a preceding direct object, which is not happening here.

Pendant des heures means for hours.

• pendant = during / for
• des heures = hours

Here it expresses duration:

• courir pendant des heures = to run for hours

This suggests a long continuous amount of time.

Also, des heures here is indefinite. It does not mean specific known hours; it means hours and hours, an unspecified long time.

In French, pendant is the normal choice to express how long something lasts in a completed or described event.

• Il a couru pendant deux heures. = He ran for two hours.

Using pour in this sentence would not sound as natural for simple duration.

So in:

• comme s’il avait couru pendant des heures

pendant is the expected word for for hours.

No. It means that he seemed so tired that it was as if he had run for hours.

That is the key meaning of comme si. It introduces an appearance or impression, not necessarily a fact.

So the sentence suggests:

• the puppy was very tired
• his tiredness was comparable to that of a puppy who had run for hours

It does not confirm that he actually ran for hours.

The sentence can be broken into three parts:

1. Après la promenade
   = after the walk

2. le chiot s’est endormi
   = the puppy fell asleep

3. comme s’il avait couru pendant des heures
   = as if he had run for hours

So the full structure is:

Time expression + main clause + comparison introduced by comme si

That makes it a very useful model sentence for learners. You can build similar sentences like:

• Après le dîner, il s’est couché comme s’il n’avait pas dormi depuis des jours.
• Après le match, elle respirait comme si elle avait couru un marathon.

A fairly literal version would be:

After the walk, the puppy fell asleep as if he had run for hours.

A more natural English rendering might also be:

• After the walk, the puppy fell asleep as though he’d been running for hours.
• After the walk, the puppy fell asleep as if he’d run for hours.

The French original is perfectly natural and paints a vivid picture of how exhausted the puppy was.