Quand l’appartement est trop bruyant, Marie travaille soit à la bibliothèque, soit chez sa sœur.

In this sentence, quand is best understood as whenever as well as when. Because the verb is in the present tense and describes a repeated situation, the idea is habitual:

• each time the apartment is too noisy,
• Marie works in one of those two places.

So an English speaker often hears a sense of whenever here, even though French still uses quand.

The part beginning with quand is a subordinate clause placed first in the sentence. French often separates that kind of introductory clause from the main clause with a comma.

So the structure is:

• Quand l’appartement est trop bruyant, = background situation
• Marie travaille... = main information

You may sometimes see short sentences without a comma, but here the comma is very natural and helps readability.

Because appartement begins with a vowel, le becomes l’. This is called elision.

Examples:

• le livre
• l’appartement
• l’hôtel

French generally avoids saying le directly before a vowel sound in this kind of case.

Because bruyant has to agree with appartement, and appartement is masculine singular.

So:

• un appartement bruyant
• l’appartement est trop bruyant

If the noun were feminine, you would use bruyante:

• la rue est trop bruyante

This is an important difference:

• très bruyant = very noisy
• trop bruyant = too noisy

So trop means the noise goes beyond what is acceptable or comfortable. It is not just strong description; it suggests excess.

Travaille is the third-person singular present of travailler.

French uses the present tense for habitual actions very often, just as English does in sentences like Marie works at the library when... or Whenever the apartment is noisy, Marie works...

So the present tense here expresses a regular pattern, not just what she is doing right now.

Soit..., soit... means either...or...

In this sentence, it introduces two alternatives:

• soit à la bibliothèque
• soit chez sa sœur

A few useful points:

• The word is normally repeated before each option.
• It is a standard, neat way to present alternatives.
• It can sound a little more structured or formal than plain ou.

So the pattern is:

• soit X, soit Y = either X or Y

Because French uses different prepositions for different kinds of places.

• à la bibliothèque: à is used for going to / being at many places or institutions.
• chez sa sœur: chez is used for at someone’s home or at someone’s place.

So:

• à la bibliothèque = at the library
• chez sa sœur = at her sister’s house / place

You would not normally say à sa sœur with that meaning.

Because the possessive adjective agrees with the noun that follows, not with the owner.

Here:

• sœur is feminine singular
• so the possessive is sa

That means:

• sa sœur = his sister or her sister, depending on context

This is often surprising for English speakers. French is not choosing sa because Marie is female; it is choosing sa because sœur is a feminine noun.

One extra detail:

• before a feminine noun beginning with a vowel or mute h, French often uses son for pronunciation reasons:
  • son amie
  • son histoire

But sœur begins with a consonant sound, so sa sœur is correct.

Yes. French allows both orders.

You could also say:

• Marie travaille soit à la bibliothèque, soit chez sa sœur quand l’appartement est trop bruyant.

Putting quand l’appartement est trop bruyant first gives the sentence a clearer context first, main action second structure. That is often the most natural choice when you want to set up the situation before stating what happens.

Chez basically means at the home/place of a person, or sometimes at the business/practice of a person.

Examples:

• chez Marie = at Marie’s place
• chez le médecin = at the doctor’s office
• chez mes parents = at my parents’ house

So in chez sa sœur, the idea is not just physical location, but specifically her sister’s place.