Près du marché, une mère attache son enfant dans le siège auto avant de monter en voiture.

Because près is used with de: près de = near.

Then French makes a contraction:

• de + le = du

So:

• près de le marché becomes près du marché

Compare:

• près de la gare
• près de l’école
• près des magasins

So près le marché is not correct here.

French often puts a place expression at the beginning of the sentence to set the scene.

So Près du marché gives the location first, then the main action comes after:

• Près du marché, une mère...

This is very natural in both French and English.
The comma marks that introductory phrase. In short sentences, the comma can sometimes be omitted, but using it here is normal and clear.

Une mère means a mother, so the sentence is introducing her as new information.

French uses:

• un / une for something not yet identified
• le / la for something already known or specific

So:

• une mère = some mother, a mother
• la mère = the mother, a specific mother already known from context

Here, the sentence is just describing a scene, so une mère fits best.

Because son / sa / ses agree with the thing possessed, not with the owner.

Here, enfant is grammatically masculine singular, so French uses son:

• son enfant

It does not matter that the owner is a mother.

A useful reminder:

• son fils = his/her son
• sa fille = his/her daughter

Also, with feminine singular nouns beginning with a vowel, French often uses son for pronunciation reasons, but in this sentence the main point is that enfant is masculine.

Here attache means fastens or buckles in.

The verb is attacher, which can mean:

• to attach
• to tie
• to fasten

In the context of a child and a car seat, it means the mother is securing the child in the seat, probably with the straps or seat belt.

So it is not just a general idea of attaching something; it specifically suggests strapping the child in safely.

Because the child is positioned in the car seat.

• dans = in / inside
• le siège auto = the car seat, more specifically a child car seat

French sees the child as being placed in the seat, so dans is the natural preposition.

Also, siège auto is a common way to say car seat for a child.

French often uses the definite article when the object is clear from the situation.

Here, there is an obvious, relevant car seat, so le siège auto sounds natural:

• dans le siège auto

Possible alternatives:

• un siège auto would sound less specific, more like in a car seat
• son siège auto would emphasize that it is his/her car seat

So le siège auto works well because the context already makes the seat identifiable.

Because after avant followed by a verb in the infinitive, French uses de:

• avant de + infinitive

So:

• avant de monter = before getting in

This structure is especially used when the subject is the same for both actions.
Here, the mother:

1. fastens the child in
2. gets into the car

If the subject changes, French usually uses avant que instead:

• avant que la voiture ne parte

Monter en voiture is a common idiomatic expression meaning to get into a car.

So in this sentence, it means the mother gets in the car after fastening the child in.

You can also say monter dans la voiture, which is more literal and physical: to get into the car.
The difference is small here:

• monter en voiture = very natural, idiomatic
• monter dans la voiture = also correct, a bit more literal

It can mean either one, depending on context.

French present tense often covers both:

• simple present: she fastens
• present progressive: she is fastening

In a scene-description sentence like this, the natural reading is usually that the action is happening right now.

If French wants to stress the ongoing action more clearly, it can use:

• est en train d’attacher

But in normal French, the simple present is often enough.

Yes, en voiture can mean by car in other contexts:

• Je voyage en voiture = I travel by car

But with monter, the meaning changes naturally:

• monter en voiture = get into the car

So the verb helps determine the meaning.
In this sentence, avant de monter en voiture clearly means before getting into the car, not before traveling by car.