El cartell de la cantonada diu que el museu és a l'esquerra.

Cartell can mean either a sign or a poster, depending on context. In this sentence, it most naturally means a sign—specifically some kind of posted sign giving directions.

So here El cartell de la cantonada is best understood as the sign on the corner or the sign at the corner.

Cantonada means corner.

In this sentence, it refers to a street corner, not just any geometric corner. So la cantonada is the corner.

Literally, de la cantonada means of the corner, but that is not the most natural way to translate it into English.

Here, the phrase is used to identify which sign we mean:

• el cartell de la cantonada = the sign on the corner / the corner sign

Catalan often uses de + noun where English might prefer:

• on the corner
• at the corner
• or an adjective-like phrase such as corner sign

So this is normal Catalan structure, not a word-for-word match with English.

Diu is the 3rd person singular present tense of the verb dir, which means to say.

So:

• dir = to say
• diu = he/she/it says

In this sentence:

• El cartell ... diu = The sign ... says

Even though a sign is not a person, Catalan uses diu just like English uses says with things like signs, books, or messages.

Because que introduces the clause that comes after diu.

So:

• diu que... = says that...

In Catalan, after verbs like dir (to say), que is very common and usually required when introducing a full clause.

So:

• El cartell diu que el museu és a l'esquerra
• literally: The sign says that the museum is on the left

English sometimes drops that, but Catalan normally keeps que here.

Because Catalan usually uses the definite article with specific nouns more regularly than English does.

Here el museu means the museum, referring to a specific museum that both speaker and listener can identify from the context.

So:

• el museu = the museum

In directions and everyday speech, Catalan often keeps the article where English might sometimes sound a bit looser.

In this sentence, és is the normal choice because the sentence is giving a location as part of a description or direction:

• el museu és a l'esquerra = the museum is on the left

In Catalan, ser is very often used for location in cases like:

• fixed location
• geographic position
• directions
• identifying where something is

So expressions like these are very natural:

• La farmàcia és aquí
• El lavabo és al final del passadís
• El museu és a l'esquerra

You may hear estar in some contexts too, but és is perfectly normal and very standard here.

A l'esquerra means on the left or to the left.

Breakdown:

• a = at / to / on
• l' = the article the
• esquerra = left

So:

• és a l'esquerra = is on the left / is to the left

This is a very common way to give directions in Catalan.

Because la becomes l' before a vowel.

Since esquerra begins with a vowel sound, the feminine singular article la is apostrophized:

• la esquerra → l'esquerra

This is the same kind of thing you see in:

• l'escola
• l'amiga
• l'hora

So a l'esquerra is the correct form.

Because al is only the contraction of:

• a + el = al

But esquerra is a feminine noun, so its article is la, not el.

With la, Catalan does not make al. Instead:

• a + la stays a la
• and if la apostrophizes before a vowel, you get a l'...

So:

• a + la esquerra → a l'esquerra

That is why al'esquerra is not correct.

The accent in és is important for two reasons:

1. It distinguishes és from es

   • és = is
   • es = a reflexive or object pronoun, like himself/herself/itself in some contexts

2. It marks pronunciation

So in writing, és clearly tells you that this is the verb to be.

A helpful way to break it up is:

• El cartell de la cantonada = The sign on the corner
• diu que = says that
• el museu = the museum
• és a l'esquerra = is on the left

So the structure is:

[subject] + [verb] + que + [new clause]

That is a very common Catalan pattern:

• La Maria diu que vindrà
• El professor diu que és fàcil
• El cartell diu que el museu és a l'esquerra

Yes. A few natural alternatives are possible, depending on style:

• El cartell de la cantonada indica que el museu és a l'esquerra.

  • The sign on the corner indicates that the museum is on the left.

• Segons el cartell de la cantonada, el museu és a l'esquerra.

  • According to the sign on the corner, the museum is on the left.

• El museu és a l'esquerra, segons el cartell de la cantonada.

  • The museum is on the left, according to the sign on the corner.

But the original sentence is completely natural and straightforward.