Avant de sortir, elle met un peu de rouge à lèvres et du mascara.

Because avant de + infinitive means before doing something.

• avant = before
• de sortir = going out / to go out

So avant de sortir means before going out.

This is a very common pattern in French:

• Avant de manger = before eating
• Avant de partir = before leaving
• Avant de dormir = before sleeping

If a full clause follows, French usually uses avant que instead:

• Avant qu’elle sorte = before she goes out

But here the subject is understood, so French uses avant de + infinitive.

After avant de, French uses the infinitive.

So:

• avant de sortir = before going out

not

• avant de elle sort ❌

This is similar to English before going out, where going is not the main conjugated verb of the sentence.

The main conjugated verb in the sentence is met:

• elle met = she puts on / applies

Yes, mettre literally means to put, but in French it is also very commonly used for putting on things, including makeup.

So:

• elle met du mascara = she puts on / applies mascara
• elle met du rouge à lèvres = she puts on / applies lipstick

English often prefers apply for makeup, but French often simply uses mettre.

You may also hear more specific verbs in some contexts, but mettre is very natural and common here.

Un peu de means a little bit of or some.

So:

• un peu de rouge à lèvres = a little lipstick / some lipstick

This expression is followed by de:

• un peu de café
• un peu d’eau
• un peu de mascara

Here it suggests she is putting on a small amount, not necessarily emphasizing the object as a whole item.

Because these are two different structures.

1. un peu de + noun

After un peu, French uses de:

• un peu de rouge à lèvres
• un peu de sucre
• un peu d’argent

2. Partitive article: du / de la / de l’

When you mean some of an uncountable substance, French often uses the partitive article:

• du mascara
• du pain
• de la confiture
• de l’eau

So in this sentence:

• un peu de rouge à lèvres = a little lipstick
• du mascara = some mascara

Both are natural, but the grammar is different.

In French, when talking about an unspecified amount of something uncountable, you usually need a partitive article.

So French says:

• du mascara
• du lait
• de la crème
• de l’eau

English often uses no article in these cases:

• mascara
• milk
• cream
• water

That is why du mascara sounds natural in French, while leaving out the article would usually sound incomplete here.

Rouge à lèvres is a fixed noun phrase meaning lipstick.

Literally, it comes from the idea of color for the lips:

• rouge = red / makeup color
• à = for / used for
• lèvres = lips

Even though it contains several words, together it functions as one noun:

• du rouge à lèvres
• un rouge à lèvres
• des rouges à lèvres

In this sentence, rouge à lèvres refers to the substance/product, so un peu de rouge à lèvres means a little lipstick.

In expressions like this, à often means for or used for.

So rouge à lèvres is roughly:

• lip-color
• literally red/color for lips

French has many fixed expressions like this:

• brosse à dents = toothbrush
• verre à eau = water glass
• machine à laver = washing machine

So à is not random here; it is part of a common French pattern.

Because a person normally has two lips, and French uses the plural in this expression.

So:

• la lèvre = the lip
• les lèvres = the lips

That is why the fixed expression is rouge à lèvres, not rouge à lèvre.

The present tense in French can describe:

• a habitual action
• a general truth
• an action in a narrative
• something happening now, depending on context

Here elle met could mean something like:

• Before going out, she puts on a little lipstick and mascara
• or Before going out, she is putting on a little lipstick and mascara

French often uses the simple present where English might choose either puts on or is putting on, depending on context.

Because the subject is elle = she, and mettre is conjugated accordingly in the present tense.

Present tense of mettre:

• je mets
• tu mets
• il / elle / on met
• nous mettons
• vous mettez
• ils / elles mettent

So:

• elle met = she puts on

The comma is natural and helpful, though punctuation can vary a little depending on style.

The opening phrase Avant de sortir sets the scene, so French often separates it with a comma:

• Avant de sortir, elle met...

This is similar to English:

• Before going out, she puts on...

It makes the sentence easier to read, especially because the introductory phrase comes first.

Yes, but the structure is different.

Avant de sortir

This is used when the subject is the same or understood:

• Avant de sortir, elle met du mascara.

Avant qu’elle sorte

This introduces a full clause with its own subject:

• Avant qu’elle sorte, elle téléphone à sa sœur.

Also, after avant que, French normally uses the subjunctive:

• avant qu’elle sorte

So both are possible in French, but they are not interchangeable in every sentence. Here, avant de sortir is the simpler and most natural form.