Learn Hardcore Greek: Παρακαλώ, θέλω νερό.

Παρακαλώ: literally “I beg/please” (1st person singular present), used as the politeness marker “please” and also “you’re welcome.”
θέλω: “I want” (1st person singular present of the verb “to want”).
νερό: “water,” a neuter noun.

Is starting with Παρακαλώ natural, or is it better to put it at the end?

Both are fine. In everyday requests, many speakers place it at the end: Θέλω νερό, παρακαλώ. Starting with it (Παρακαλώ, θέλω νερό.) is also natural, often to get someone’s attention politely.

Is θέλω too direct? What are more polite alternatives?

Θέλω isn’t rude, but it is direct. Softer options:

Θα ήθελα νερό. (I would like water.)
Μπορώ να έχω λίγο νερό; (May I have some water?)
Ένα νερό, παρακαλώ. (One water, please — typical when ordering.)

Do I need the article before νερό? When do I use το?

For water in general, no article: Θέλω νερό. Use το when referring to a specific water: Θέλω το νερό που άφησες. When ordering a unit as a product, you may hear ένα νερό (“one water” = one bottle/glass).

How do I say “some water” or “a water”?

“Some water”: λίγο νερό.
“A water” (in cafés/restaurants, meaning one bottled/glass water): ένα νερό.

How do I pronounce the sentence?

pa-ra-ka-LO, THEH-lo ne-RO.

θ = th in “think”
ρ is a tapped/rolled r
Stress is where the accent mark is: παρακαΛΩ, ΘΕλω, νεΡΟ
ε like e in “bet”; ο/ω like short o in “not.”

What gender is νερό, and what’s its plural?

νερό is neuter. Singular: το νερό. Plural: τα νερά (used for “waters” as types, or colloquially for multiple bottles: δύο νερά = two waters/bottles).

How do I make it negative?

Put δεν before the verb: Δεν θέλω νερό, ευχαριστώ. (“I don’t want water, thanks.”)

How can I turn it into a polite question?

Μπορώ να έχω λίγο νερό; (May I have some water?)
Θα μπορούσα να έχω λίγο νερό; (Could I have some water?)
Ένα ποτήρι νερό, παρακαλώ; (A glass of water, please?) Remember: the Greek question mark is the semicolon character: ;

Is the comma after Παρακαλώ required?

It’s optional. It simply separates the interjection from the main clause. You’ll also see Θέλω νερό, παρακαλώ. or Παρακαλώ! Θέλω νερό. — all acceptable.

Does Παρακαλώ also mean “you’re welcome”?

Yes. After Ευχαριστώ (“thank you”), you reply Παρακαλώ (“you’re welcome”). You’ll also hear Παρακαλώ; on the phone, meaning “Yes, please?/How can I help you?”

How do I say this with different subjects (I/you/we, etc.)?

I want: θέλω
You (sing.) want: θέλεις
He/She/It wants: θέλει
We want: θέλουμε
You (pl./pol.) want: θέλετε
They want: θέλουν(ε) Example: Θέλουμε νερό. (We want water.)

Is there a difference between νερό and ύδωρ?

νερό is the normal modern word. ύδωρ is archaic/learned; you’ll mostly see it in compounds (e.g., υδρο- words) or very formal contexts.

How can I specify tap, bottled, or sparkling water?

Tap water: νερό βρύσης
Bottled water: εμφιαλωμένο νερό
Sparkling water: ανθρακούχο νερό (or νερό με ανθρακικό)