Quiero un refresco.

Because Spanish often omits the subject pronoun when it is already clear from the verb form.

Quiero already means I want, so yo is not necessary.
You would add yo only for emphasis, contrast, or clarity, for example:

Yo quiero un refresco, pero él quiere agua.
= I want a soft drink, but he wants water.

So Quiero un refresco is the most natural neutral version.

Because refresco is a masculine singular noun.

In Spanish:

• un = a / an for masculine singular nouns
• una = a / an for feminine singular nouns

So:

• un refresco
• una bebida

A lot of nouns ending in -o are masculine, and refresco is one of them.

Because querer is a stem-changing verb.

Its stem changes from e to ie in most present-tense forms:

• yo quiero
• tú quieres
• él/ella quiere
• nosotros queremos
• vosotros queréis
• ellos quieren

So quiero is the correct I form of querer in the present tense.

In Spain, refresco usually means a soft drink or fizzy drink, such as cola, lemon soda, orange soda, and similar drinks.

Depending on context, it can sometimes be understood more generally as a cold soft drink, but in everyday use in Spain it most often refers to the kind of drink you would order in a bar, café, or restaurant instead of water, beer, wine, etc.

It is grammatically correct and people will understand it perfectly, but it can sound a bit direct, because quiero is literally I want.

In Spain, this is not automatically rude, especially in casual situations, but if you want to sound more polite, you could say:

• Quería un refresco, por favor.
• Me gustaría un refresco, por favor.
• Querría un refresco, por favor.

So Quiero un refresco is fine, but adding por favor or using a softer structure is often better in service situations.

Both can be used to ask for a soft drink, but the tone is different.

• Quiero un refresco = more direct
• Me gustaría un refresco = softer, more polite, more like I would like a soft drink

English often uses I’d like... in polite requests, and Spanish often does the same with me gustaría.

So if you are ordering in a bar or restaurant, Me gustaría un refresco, por favor is often the safest polite choice.

A simple English-friendly approximation is:

KYEH-ro oon reh-FRES-ko

A few useful pronunciation points:

• qu before e or i sounds like k
• the u in que / qui is normally silent
• quiero is roughly KYEH-ro
• refresco has the stress on -fres-
• the c before o sounds like k

So:

• quiero → KYEH-ro
• refresco → reh-FRES-ko

Usually, no. If you mean a soft drink, Spanish normally needs the article:

Quiero un refresco.

Without un, it sounds less natural in standard Spanish. It can sound incomplete or unnatural in this context.

Compare:

• Quiero agua. → natural, because agua is often treated as an uncountable substance
• Quiero un refresco. → natural, because you usually mean one drink

So with refresco, using un is the normal choice.

The normal word order here is:

Quiero + un refresco

That is the most natural neutral order in Spanish.

If you change it to something like Un refresco quiero, it sounds marked or unnatural unless you are emphasizing un refresco very strongly, perhaps in a special context.

So for normal everyday speech, stick with:

Quiero un refresco.

You do not need to for the sentence to be correct, but socially it is often a good idea.

So:

• Quiero un refresco. = correct
• Quiero un refresco, por favor. = more polite

If you are speaking to staff in a café, bar, or restaurant, adding por favor makes the request sound more natural and courteous.