Posa un tovalló al costat del plat, si us plau.

Posa is the tu imperative of posar, meaning to put or to place. It is used when giving a command to one person informally.

So this sentence is speaking to someone as tu:

• (tu) posa = put
• (vostè) posi = put, formal singular
• (vosaltres) poseu = put, informal plural

Catalan often leaves out the subject pronoun, so Posa by itself is completely normal.

Un is the masculine singular indefinite article, so it means a or one.

• un tovalló = a napkin
• el tovalló = the napkin

Here, un tovalló sounds natural because you are asking for a napkin, not pointing to one specific napkin already identified.

Yes. Tovalló is the standard Catalan word for napkin.

You may also hear servilleta in some places or in more colloquial speech, but tovalló is a good standard word to learn and use.

It is built from the expression al costat de, which means next to or beside.

Breakdown:

• al = a + el
• costat = side
• de = of
• del = de + el
• plat = plate

So literally it is something like at the side of the plate, which in natural English is next to the plate.

Because costat normally takes de before the thing you are next to:

• al costat de la tassa = next to the cup
• al costat de la porta = next to the door
• al costat del plat = next to the plate

Since de + el contracts to del, you get del plat.

Catalan uses definite articles more often than English in this kind of phrase. In English, we say next to the plate, and Catalan also wants that definite article idea:

• del plat = of the plate / next to the plate

Leaving out the article here would sound unnatural in standard Catalan.

Yes. That is completely normal.

Si us plau simply adds politeness. It does not force the rest of the sentence to be formal. So you can say:

• Posa un tovalló al costat del plat, si us plau.
  informal but polite

And the formal version would be:

• Posi un tovalló al costat del plat, si us plau.

Literally, it comes from something like if it pleases you. But in modern Catalan, you should mostly treat it as a fixed expression meaning please.

Also, even though it contains us, it is commonly used as a general polite expression and does not necessarily mean you are speaking to several people.

Most commonly, it goes at the beginning or end:

• Si us plau, posa un tovalló al costat del plat.
• Posa un tovalló al costat del plat, si us plau.

For learners, those are the safest patterns to use. The version with si us plau at the end is especially common in everyday speech.

Here are the main alternatives:

• Posa un tovalló al costat del plat, si us plau.
  informal singular

• Posi un tovalló al costat del plat, si us plau.
  formal singular

• Poseu un tovalló al costat del plat, si us plau.
  informal plural

The rest of the sentence stays the same; only the verb changes.

The accent marks the stressed syllable, so the stress falls on the last syllable:

• to-va-lló

That means you should not stress the first syllable. The final ó is the stressed part.

A rough Central Catalan pronunciation is:

[ˈpɔ.zə un tu.βəˈʎo əl kəsˈtat dəl ˈplat, si us ˈplaw]

A simple English-friendly approximation would be:

POH-zuh oon too-buh-LYOH uhl kuh-STAT del PLAT, see oos PLOW

A few helpful notes:

• ll in tovalló is a palatal sound, similar to lli in some pronunciations of million
• al and del are pronounced as short unstressed words
• plau rhymes roughly with cow