Demà només vull una amanida i arròs.

Catalan often uses the present tense to talk about the future when the time is already clear from a word like demà.

So Demà només vull una amanida i arròs is perfectly natural.

You could also use the future:

• Demà només voldré una amanida i arròs

But that sounds a bit more marked or formal here. In everyday speech, the present is very common.

Vull is the 1st person singular present of voler, meaning to want.

So:

• vull = I want

This verb is irregular, so it does not follow a completely regular pattern.

A few useful present-tense forms are:

• vull = I want
• vols = you want
• vol = he/she/it wants
• volem = we want
• voleu = you all want
• volen = they want

Catalan often drops subject pronouns when they are not needed, because the verb form already shows who the subject is.

So:

• vull already tells you the subject is I

You could say:

• Jo demà només vull una amanida i arròs

But jo is usually added only for emphasis, contrast, or clarity.

Només means only or just.

In this sentence, it limits what the speaker wants:

• Demà només vull una amanida i arròs
  = Tomorrow I only want / just want a salad and rice.

Putting només before the verb is a very normal position in Catalan.

Its position can affect emphasis:

• Només demà vull... = I want ... only tomorrow
• Demà només vull... = tomorrow, I only want ...

So the placement here is the most natural one for the intended meaning.

Because amanida is a countable noun, while arròs here is being used as a mass noun.

• una amanida = a salad
  You can count salads: one salad, two salads.
• arròs = rice / some rice
  In this sentence, it means rice in a general, uncounted sense.

In Catalan, singular countable nouns usually need a determiner such as un / una / el / la, but mass nouns often do not.

So:

• una amanida is required
• arròs without an article is natural

Here, arròs is singular.

In Catalan, some singular nouns naturally end in -s, so an -s at the end does not always mean plural.

• arròs = rice (singular/mass noun)

If you were talking about different rice dishes or types, you might see a plural such as arrossos, but in this sentence arròs is just singular mass-noun rice.

• amanida is feminine, which is why it takes una
• arròs is masculine in dictionary form: l'arròs

Even though arròs is masculine, there is no article here because it is being used as an uncounted mass noun.

So:

• una amanida
• arròs

If you used the definite article, you would get:

• l'arròs = the rice

Yes. Catalan word order is fairly flexible, especially with time expressions like demà.

The version you have is very natural:

• Demà només vull una amanida i arròs

But you could also hear:

• Només vull una amanida i arròs demà
• Demà vull només una amanida i arròs

These alternatives may sound slightly different in rhythm or emphasis, but the basic meaning is similar. Putting demà first is very common because it sets the time frame right away.

The accents help with stress, and sometimes also with vowel quality.

• demà: the stress is on the last syllable
• només: the stress is on the second syllable
• arròs: the stress is on the last syllable

In Catalan, accents on e and o can also show whether the vowel is more open or closed:

• é in només
• ò in arròs

So the accents are not optional: they are part of the correct spelling and pronunciation.

Not usually if you simply mean a salad and some rice.

• arròs by itself normally means rice as food in general
• un arròs often suggests a rice dish or a type/serving of rice, especially in cooking contexts

So:

• una amanida i arròs = a salad and rice
• una amanida i un arròs = more like a salad and a rice dish

That is why the original sentence sounds the most natural for the usual meaning.