Si hi ha molta cua, seu en aquesta cadira i espera el teu torn.

Si means if.

So Si hi ha molta cua... means If there is a long queue / if there are a lot of people waiting...

A useful thing to remember is:

• si = if
• sí (with an accent) = yes

Those are different words.

Hi ha means there is or there are.

It comes from the verb haver in this fixed expression:

• hi ha una cadira = there is a chair
• hi ha cadires = there are chairs

Why two words?

• ha is the verb form
• hi is a little adverb-like word that is part of this existence expression

You should learn hi ha as a set phrase meaning there is / there are.

In Catalan, hi ha does not change between singular and plural in this use.

So:

• hi ha una persona = there is one person
• hi ha moltes persones = there are many people

English changes between is and are, but Catalan keeps hi ha the same.

Cua literally means tail, but it also commonly means queue, line, or a lot of waiting people.

So molta cua means:

• a long queue
• a lot of line / waiting
• more naturally in English, a long line

Molta is the feminine singular form of much / a lot of, because cua is feminine singular.

Compare:

• molta cua = a long queue / a lot of queue
• molts clients = many customers
• moltes persones = many people

Because cua is a feminine singular noun.

In Catalan, words like molt change to agree with the noun:

• molt = masculine singular
• molta = feminine singular
• molts = masculine plural
• moltes = feminine plural

So:

• molt temps = a lot of time
• molta cua = a long queue
• molts llibres = many books
• moltes cadires = many chairs

Seu here is the informal singular imperative of seure, which means to sit or to sit down.

So seu means:

• sit
• sit down

The sentence is giving an instruction to one person informally:

• Seu en aquesta cadira = Sit in this chair

This is not the possessive seu / seva meaning his/her/their. It is a verb form here.

Because the sentence is a command.

• seu = sit down! (imperative)
• s'asseu = he/she sits down or is sitting down (present tense, depending on context)

So:

• Seu! = Sit down!
• Ell s'asseu. = He sits down / He is sitting down.

In your sentence, the speaker is telling someone what to do, so the imperative seu is correct.

Yes. Here en means in / on, depending on the context.

With seats, Catalan often uses en where English might say in or on:

• Seu en aquesta cadira = Sit in/on this chair

This is normal Catalan usage.

In everyday Catalan, especially depending on region and style, you may also hear a in some contexts, but en aquesta cadira is perfectly natural.

Aquesta means this.

So:

• aquesta cadira = this chair

Catalan does not need an extra word like here if the demonstrative already makes it clear.
So aquesta cadira already means the chair near the speaker.

Compare:

• aquesta cadira = this chair
• aquella cadira = that chair over there

Because this is also an informal singular command.

For esperar:

• tu esperes = you wait (statement)
• espera! = wait! (command)

So in the sentence:

• espera el teu torn = wait for your turn

Catalan commands often look like the third person singular present form for tu commands:

• parles → parla!
• mires → mira!
• esperes → espera!

Because subject pronouns are often omitted in Catalan when they are not needed.

The verb form already shows that this is directed at tu:

• seu
• espera

So tu is understood.

You can add tu for emphasis, but normally it is left out.

El teu torn means your turn.

Parts:

• el = the
• teu = your (masculine singular)
• torn = turn

So literally it is the your turn, but that sounds wrong in English.
In Catalan, possessives are very often used with the definite article:

• el meu llibre = my book
• la teva cadira = your chair
• el teu torn = your turn

This is a very common feature of Catalan.

Catalan often uses:

definite article + possessive + noun

So:

• el meu pare = my father
• la seva feina = his/her job
• el teu torn = your turn

English usually does not use the in these cases, but Catalan often does.

Because torn is a masculine singular noun.

The possessive must agree with the noun:

• el teu torn = masculine singular
• la teva cadira = feminine singular
• els teus llibres = masculine plural
• les teves coses = feminine plural

So teu matches torn.

In natural English, it means wait your turn or wait for your turn.

Catalan uses esperar directly with the thing awaited:

• esperar el bus = wait for the bus
• esperar una resposta = wait for an answer
• esperar el teu torn = wait for your turn

So Catalan does not need a separate word equivalent to English for here.

Because the sentence begins with a conditional clause:

• Si hi ha molta cua = If there is a long queue

Then comes the main instruction:

• seu en aquesta cadira i espera el teu torn

The comma helps separate the if-clause from the main clause, just as in English:

• If there’s a long line, sit in this chair and wait your turn.

It is informal singular.

The commands seu and espera are forms used for tu.

If the speaker were addressing someone formally (vostè), the commands would be different, for example:

• segui
• esperi

So this sentence is talking to one person in a normal, informal way.

Yes. Literally, cua means tail.

But in many everyday contexts, it also means:

• queue
• line

So the meaning depends on context.

Examples:

• El gos mou la cua. = The dog wags its tail.
• Hi ha molta cua al supermercat. = There is a long line at the supermarket.

In your sentence, it clearly means queue / line.

It has a very common pattern:

Si + condition, + command(s)

Breakdown:

• Si hi ha molta cua = condition
• seu en aquesta cadira = first command
• i espera el teu torn = second command

So the whole sentence is built like:

If X happens, do Y and Z.

That is a very useful pattern in Catalan.