Hi ha un cartell a l'entrada del museu que diu que avui tanquen tard.

Hi ha is the normal Catalan way to say there is or there are. It is used to say that something exists or is present somewhere.

So Hi ha un cartell means There is a sign.

You would not use és or està here because those mean different things. És is used for identity or description, and està is used for location or temporary state. In this sentence, the idea is first introducing the existence of the sign, so hi ha is the right choice.

Cartell usually means a sign, poster, or notice with written information on it.

In this sentence, it most naturally means a posted notice or sign at the museum entrance.

A learner may wonder about senyal, which can also mean sign, but senyal is more often a signal or sign in the sense of a marker, symbol, or traffic sign. For a written notice, cartell is very natural.

Because entrada is a feminine noun, so the full form of the article is la. But before a vowel, la becomes l'.

So:

la entrada → l'entrada

Then you add the preposition a:

a + l'entrada = a l'entrada

Here it means at the entrance.

Del means of the.

It is formed by combining:

de + el = del

So:

del museu = of the museum

This contraction happens with de + el, but not with feminine la. For example, you would say de la biblioteca, not a contraction.

The two que words do different jobs.

The first que refers back to un cartell. It means that or which in the sense of a relative clause:

un cartell que diu... = a sign that says...

The second que introduces what the sign says:

diu que avui tanquen tard = says that today they close late

So the first que links the sign to its description, and the second que introduces the content of the message.

Catalan often leaves out subject pronouns because the verb ending usually makes the person clear.

In que diu, the subject is already understood to be un cartell.

In avui tanquen tard, the subject is not stated, but Catalan allows that. The verb form tanquen already tells you it is third person plural, so the listener understands something like they close.

This is very common in Catalan and Spanish, but less common in English, where the pronoun usually has to be said.

Tanquen is the third person plural form of tancar, meaning they close.

In context, it refers to the people or institution running the museum. English often does something similar with they when the exact subject is not named.

So the idea is something like:

today they close late or today the museum closes late

Catalan can leave that plural subject unstated if it is obvious from context.

Yes, avui tanca tard would also be possible.

The difference is mainly in perspective:

tanca treats the museum as a single entity, like saying the museum closes late today

tanquen suggests they close late today, with the people in charge understood in the background

Both are natural, but tanquen sounds a bit more like an unstated they, while tanca points more directly to the museum as the thing that closes.

Because tard here is an adverb meaning late.

So:

tanquen tard = they close late

Tarda, on the other hand, is a noun meaning afternoon.

So these are different words:

tard = late
tarda = afternoon

In this sentence, it means at the entrance of the museum or at the museum entrance.

The preposition a can cover meanings that English expresses with at, to, or sometimes in, depending on context. Here, because the sentence is describing where the sign is located, the meaning is clearly at.

So a l'entrada del museu is a location phrase.

Yes, Catalan word order is fairly flexible, especially with adverbs like avui.

Avui tanquen tard is a very natural order and clearly sets the time first: Today they close late.

Other orders are possible, but they may sound slightly different in emphasis. Putting avui near the beginning is often the clearest and most neutral choice.