Hem de decidir avui si anem al cinema o al restaurant.

Because Catalan uses haver de + infinitive to express obligation.

• hem = we have
• de = part of the obligation structure
• decidir = to decide

So hem de decidir means we have to decide or we must decide.

Even though English uses have to, standard Catalan normally uses haver de, not tenir.

So:

• Hem de decidir = correct, standard Catalan
• Tenim de decidir = possible in some varieties, but not the usual standard choice
• Tenim que decidir = generally avoided in standard Catalan

This is one of those places where Catalan and English do not match word-for-word.

Catalan often leaves out subject pronouns because the verb ending already shows who the subject is.

Here:

• hem already tells you it means we
• anem also already means we go / we are going

So Nosaltres hem de decidir... is possible, but it is only used when you want extra emphasis or contrast.

Here si means whether.

So:

• Hem de decidir avui si anem al cinema o al restaurant
  = We have to decide today whether we’re going to the cinema or to the restaurant

This is very common in Catalan: si can mean if or whether, depending on context.

The accent changes the meaning:

• si = if / whether
• sí = yes

In this sentence, it is si without an accent because it introduces an indirect question: whether we go to the cinema or to the restaurant.

No. This si does not mean a condition like if it rains, we’ll stay home.

Here it introduces an indirect yes/no question:

• decidir si anem... = decide whether we go...

So the sentence is about making a choice, not about a condition.

Anem is the present indicative of anar for we.

Catalan often uses the present tense for a present or near-future plan, especially in everyday speech. So si anem al cinema... can naturally mean whether we go / whether we’re going to go.

Using anirem is not impossible in every context, but anem is the more natural, simple choice here.

Because Catalan, like English, often leaves out repeated words when they are understood.

The full idea is:

• si anem al cinema o (si anem) al restaurant

But repeating si anem would sound unnecessary. Once the first option is clear, the second option can just be added after o.

Al is a contraction of:

• a
  • el = al

So:

• al cinema = to the cinema
• al restaurant = to the restaurant

This contraction is normal in Catalan.

Because Catalan often uses the definite article with places after anar a.

So you say:

• anar al cinema
• anar al restaurant
• anar a la platja

English sometimes uses no article in similar expressions, but Catalan usually keeps it.

In this sentence, avui most naturally modifies decidir.

So the idea is:

• We have to decide today whether we go to the cinema or to the restaurant.

It does not mainly mean that the going happens today. The word order helps make that clear.

Yes, Catalan word order is fairly flexible.

For example:

• Avui hem de decidir si anem al cinema o al restaurant.
• Hem de decidir avui si anem al cinema o al restaurant.

Both are natural. The version with avui after decidir is very clear because it strongly suggests that today refers to the decision, not necessarily to the outing itself.