L'armariet és al vestidor.

Because armariet begins with a vowel sound. In Catalan, the masculine singular article el is usually shortened to l' before a vowel:

• el llibre = the book
• l'armariet = the small cabinet / the locker

So el armariet would sound wrong; the correct form is l'armariet.

Armariet is a diminutive form of armari.

• armari = wardrobe / cabinet / cupboard
• armariet = small cabinet / little cupboard / locker

In this sentence, because it is in a vestidor, armariet very likely means locker.

The accent distinguishes és from es.

• és = is (from the verb ser)
• es = a reflexive pronoun, like oneself / himself / herself, depending on context

So the accent is important both for spelling and meaning.

Catalan often uses ser for location in sentences like this, especially when giving the normal or identifying location of something.

So:

• L'armariet és al vestidor. = The locker is in the changing room.

In everyday speech, some speakers may also use està in certain contexts, but és is completely normal here. A good learner rule is that Catalan does not match English is/are vs. is located in a simple one-to-one way.

Al is a contraction of a + el.

• a = to / at / in depending on context
• el = the
• al = to the / at the / in the

In this sentence, al vestidor means in the changing room or at the changing room, depending on how naturally you would say it in English. Here, in the changing room is the best translation.

Catalan often uses a with places where English might use in or at.

So al vestidor is very natural Catalan. English speakers often expect en because it looks like in, but Catalan does not always work that way.

Both a and en can appear with places in Catalan, but in simple location sentences like this, al vestidor is very common and idiomatic.

Vestidor can mean different but related things depending on context:

• changing room
• dressing room
• walk-in closet

In this sentence, because there is an armariet inside it, changing room or dressing room is probably the intended meaning.

Yes, it is very similar here:

• L'armariet = subject
• és = verb
• al vestidor = place/location

So the structure is basically:

The locker + is + in the changing room

That makes this sentence quite straightforward for an English speaker.

A simple stress guide is:

• armariet → ar-ma-ri-ET
• vestidor → ves-ti-DOR

And the whole sentence is roughly:

larmariet es al vestidor

The exact pronunciation varies a bit by dialect, but the main thing for learners is the stress on the last syllable of armariet and vestidor.

Because they are masculine nouns in Catalan, so they take masculine articles:

• l'armariet < el armariet
• el vestidor

There is not always a simple reason you can predict from English. You usually need to learn the noun together with its article. In this case, both are masculine.

You would change the noun, verb, and article/preposition as needed:

• L'armariet és al vestidor.
• Els armariets són al vestidor. = The lockers are in the changing room.

If both nouns are plural:

• Els armariets són als vestidors. = The lockers are in the changing rooms.

Notice:

• és → són
• l'armariet → els armariets
• al → als when it means a + els