Comme il pleut, Paul regarde si le pare-brise est propre et règle son rétroviseur avant de démarrer.

Here Comme means since or because.

In French, comme is often used at the beginning of a sentence to give the reason first:

• Comme il pleut, ... = Since it’s raining, ...

A learner might expect parce que, but parce que is more commonly used after the main clause:

• Paul regarde si le pare-brise est propre parce qu’il pleut.

Both are possible, but comme at the start is very natural when the reason is already known or obvious.

No. In il pleut, il does not refer to a person.

This is an impersonal expression:

• il pleut = it is raining
• il neige = it is snowing
• il faut = it is necessary

So the il here is just a grammatical subject, like it in English weather expressions.

Here si means whether or if in the sense of checking something.

• Paul regarde si le pare-brise est propre = Paul checks whether the windshield is clean

This is not the same as if in a conditional sentence like if it rains, we stay home.

A useful pattern is:

• regarder si... = to see/check whether...
• vérifier si... = to check whether...

So si introduces an indirect yes/no question.

Le pare-brise means windshield in American English or windscreen in British English.

The hyphen is normal because pare-brise is a fixed compound noun.

Literally, it comes from the idea of something that wards off / protects against the wind:

• parer = to ward off, protect against
• brise = breeze/wind

In modern French, you should just learn le pare-brise as one vocabulary item.

Propre is an adjective here, meaning clean.

• le pare-brise est propre = the windshield is clean

Why not proprement?

• proprement is an adverb, meaning something like cleanly or properly
• after être, you normally use an adjective, not an adverb

Why not propres?

• because pare-brise is singular
• plural would be propres

A small extra point: propre looks the same in masculine and feminine singular:

• un pare-brise propre
• une vitre propre
• plural: propres

Yes, it is the same spelling base, but here règle is the verb régler.

• régler = to adjust, set, regulate
• il/elle règle = he/she adjusts

So:

• Paul règle son rétroviseur = Paul adjusts his rear-view mirror

This is different from the noun:

• une règle = a rule / a ruler

The accent matters:

• règle can be the noun or a form of the verb
• in this sentence, it is clearly the verb because it follows Paul and works as the second action after regarde

Because rétroviseur is a masculine singular noun.

French possessive adjectives agree with the thing possessed, not with the owner.

So:

• son rétroviseur = his mirror
• sa voiture = his car
• ses clés = his keys

Even though the owner is Paul, the form depends on rétroviseur, which is masculine singular, so son is correct.

Le rétroviseur means rear-view mirror or simply mirror in a car context.

It can refer to:

• the inside rear-view mirror
• sometimes a side mirror, depending on context

In this sentence, the exact mirror is not the main point; the idea is that Paul adjusts a car mirror before driving.

After avant, French normally uses de + infinitive when the subject is the same as in the main clause.

So:

• avant de démarrer = before starting
• avant de partir = before leaving
• avant de manger = before eating

Why not avant démarrer?

• because French requires de here

Why not avant de démarre?

• because after de, you need the infinitive
• the infinitive is démarrer, not démarre

It is still Paul.

French often uses de + infinitive when the subject stays the same:

• Paul regarde..., règle..., avant de démarrer
• all three actions belong to Paul

So the sentence means:

• Paul checks the windshield
• Paul adjusts his mirror
• then Paul starts/drives off

If the subject changed, French would usually need a fuller structure.

The sentence is in the present tense because French often uses the present for:

• general description
• narration
• instructions
• habits
• events happening now

Here we have:

• pleut
• regarde
• est
• règle

This can describe a present situation very naturally:

• Since it’s raining, Paul checks whether the windshield is clean and adjusts his mirror before starting.

French uses the present in many places where English might also use the present, so this part is fairly straightforward.

Because neither place needs one.

1. propre is an adjective after être

   • est propre = is clean
   • adjectives do not take articles there

2. démarrer is an infinitive after de

   • avant de démarrer = before starting
   • infinitives used this way do not take an article

So the structure is normal French grammar, not missing words.

Yes.

In a car context, démarrer can mean:

• to start the engine
• to set off / drive away

In this sentence, either reading works well:

• Paul checks the windshield
• adjusts the mirror
• then starts the car / sets off

The exact nuance depends on context, but both are natural.