Pourriez-vous nettoyer le pare-brise avant de partir, à moins que vous préfériez attendre la fin de la pluie ?

Pourriez-vous is the conditional form of pouvoir. It makes the request more polite and less direct.

• Pouvez-vous... ? = Can you... ?
• Pourriez-vous... ? = Could you... ?

In English, could you often sounds softer than can you, and French works similarly here.

The hyphen appears because this is inversion in a yes/no question.

French often forms formal questions by putting the verb before the subject pronoun:

• Vous pourriez nettoyer... = You could clean...
• Pourriez-vous nettoyer... ? = Could you clean...?

The hyphen links the verb and the subject pronoun in this structure.

Vous can mean either:

• you plural
• you singular but polite/formal

In this sentence, without more context, it could be either. Very often in a sentence like this, it is understood as a polite singular you.

If you were speaking to one person informally, you would say:

• Pourrais-tu nettoyer le pare-brise... ?

Le pare-brise means the windshield.

It is a masculine noun, so it takes le.

Literally, pare-brise comes from the idea of something that wards off or protects against the wind. You do not need the literal breakdown to use it, but that is where the word comes from.

So:

• le pare-brise = the windshield
• un pare-brise = a windshield

After avant, if the next verb stays in the infinitive, French normally uses de:

• avant de partir = before leaving / before you leave

So the pattern is:

• avant de + infinitive

Examples:

• avant de manger = before eating
• avant de sortir = before going out

You cannot normally say avant partir.

It is understood to be the same person as in the main clause—here, the person or people being addressed.

So in this sentence, avant de partir naturally means:

• before you leave

This is a common French pattern: when the subject of both actions is the same, French often uses de + infinitive.

If the subject were different, French would usually use avant que + subjunctive instead.

For example:

• Nettoyez le pare-brise avant que Paul parte.
• Clean the windshield before Paul leaves.

À moins que means unless or except if.

In this sentence:

• à moins que vous préfériez attendre...
• unless you would prefer to wait...

It introduces an exception to the request:

• Please clean the windshield before leaving,
• unless you would rather wait until the rain is over.

Because à moins que is normally followed by the subjunctive.

So here:

• que vous préfériez

is the subjunctive form of préférer.

French uses the subjunctive after certain expressions that involve uncertainty, possibility, condition, emotion, or judgment. À moins que is one of those expressions.

That is a very good question. For many verbs, the vous form of the present subjunctive looks exactly like the imperfect form.

So vous préfériez can be:

• imperfect indicative, depending on context
• present subjunctive, depending on context

Here it is subjunctive because the grammar before it requires it:

• à moins que
  • subjunctive

So the structure tells you the mood, even if the spelling looks familiar.

You are right: in more formal French, you often see:

• à moins que vous ne préfériez...

This ne is called an expletive ne. It does not make the sentence negative. It is just a traditional grammatical feature used after certain expressions such as à moins que, avant que, de crainte que, and some others.

So these both mean the same thing:

• à moins que vous préfériez...
• à moins que vous ne préfériez...

The version without ne is very common in modern French, especially in everyday speech.

Yes, it is understandable and acceptable, but it sounds a little more literal than some alternatives.

French speakers might also say:

• attendre que la pluie s'arrête
• attendre que la pluie cesse
• attendre la fin de l'averse if it is specifically a shower

So attendre la fin de la pluie is not wrong, but depending on context, another version may sound a bit more idiomatic.

You can get close, but not perfectly. A rough literal breakdown would be:

• Pourriez-vous = Could you
• nettoyer = clean
• le pare-brise = the windshield
• avant de partir = before leaving
• à moins que = unless
• vous préfériez = you prefer / you would prefer
• attendre la fin de la pluie = wait for the end of the rain

But natural English would usually smooth this out into something like:

• Could you clean the windshield before leaving, unless you'd prefer to wait until the rain stops?

That is why literal meaning and natural meaning are not always exactly the same.