Hier, mes cousines se sont installées dans un studio plus calme au centre-ville.

It is the passé composé, the most common French tense for a completed action in the past.

Here, it shows that the action happened yesterday and is viewed as finished: they moved in / settled in yesterday.

The structure is:

subject + auxiliary + past participle

So here: mes cousines + se sont + installées

Because the verb is s’installer, it uses être as its auxiliary in the passé composé.

Because the verb here is s’installer, a pronominal verb. Pronominal verbs use a reflexive pronoun like me, te, se, nous, vous, se.

In the passé composé, pronominal verbs are normally formed with être, not avoir:

• elles se sont installées
• not elles ont installé

Also, installer and s’installer do not mean exactly the same thing:

• installer quelque chose / quelqu’un = to install something / settle someone in
• s’installer = to settle in, move in, get established

So if you say ont installé, you would usually need a direct object:

• Elles ont installé les meubles.

But here the idea is that they themselves moved in, so se sont installées is the right form.

Because it agrees with mes cousines, which is feminine plural.

With s’installer in the passé composé, the past participle agrees here with the subject:

• masculine singular: installé
• feminine singular: installée
• masculine plural: installés
• feminine plural: installées

Since mes cousines refers to more than one female person, you need installées.

Here s’installer means something like to move in, to settle in, or to get set up in a new place.

It is a little broader than just physically entering a place. It often suggests beginning to live there or becoming established there.

So in this sentence, it is more natural than a simple verb like entrer.

Yes. Cousines is specifically female cousins.

French makes this distinction:

• cousin = male cousin
• cousine = female cousin
• cousins = male cousins, or a mixed group
• cousines = all female cousins

So this sentence tells you that the cousins are female, which also explains the feminine plural agreement in installées.

Because dans is the natural preposition for being in or moving into an enclosed space or place.

Here, the studio is the place they moved into, so dans works well:

• dans un studio
• dans un appartement
• dans une maison

By contrast, French often uses à for towns, cities, or general locations:

• à Paris
• au centre-ville

So the sentence has both patterns:

• dans un studio
• au centre-ville

It is very similar. In French, un studio usually means a small one-room apartment or studio flat.

So it does not mean an art studio or recording studio here. In housing vocabulary, studio is a very common word for a compact apartment with one main room.

Because French can leave the comparison implicit when it is clear from context.

Plus calme means quieter or more quiet. The sentence does not explicitly say what the studio is quieter than, but that is normal.

If you wanted to state the full comparison, you could say:

• un studio plus calme que leur ancien logement
• un studio plus calme que le précédent

But very often French just says plus + adjective and lets the listener infer the comparison.

Because in French, many adjectives normally come after the noun, and calme is one of them.

So:

• un studio calme
• un studio plus calme

This is different from English, where adjectives usually come before the noun:

• a quiet studio
• a quieter studio

When you make the adjective comparative, it still stays after the noun:

• studio + plus calme

Centre-ville means downtown, the town center, or the city center.

Au is the contraction of à + le:

• à le → au

So:

• au centre-ville = in the city center / downtown

This happens because centre-ville is treated as a masculine singular expression:

• le centre-ville
• therefore au centre-ville

Because centre-ville is a fixed compound noun in French.

The hyphen is the standard spelling for this expression when it means downtown / city center.

French compound words are not always formed the same way, so this is mostly something to learn as vocabulary:

• centre-ville
• arc-en-ciel
• pomme de terre (no hyphen)

So here the hyphen is simply the normal spelling.

Yes, it can go somewhere else.

Putting Hier at the beginning is very common because it sets the time frame right away:

• Hier, mes cousines se sont installées...

But you could also say:

• Mes cousines se sont installées hier dans un studio plus calme au centre-ville.

Both are correct. The version with Hier first sounds a little more like you are introducing the event: Yesterday, ...

Not absolutely, but it is very common and natural in writing.

When a short time expression like Hier comes first, many writers add a comma to mark it as an introductory element:

• Hier, mes cousines...

You may also see:

• Hier mes cousines...

The comma helps readability, especially in careful writing, but the sentence would still be understandable without it.