@enteka, do we pronounce this sentence as follows:
Poyo íne aftó to koríci?

I have a question regarding the accents: I know there is an accent on every Greek word, to show where the stress falls. Yet, in this sentence, the first word is "poyo" (I don't know if I read it correctly), and there's no accent there. How do we know if it's póyo or poyó?

I believe I made a mistake in pronouncing the first word. Is it "pio"? (I wrote "poyo" in my previous comment). But there's no accent there.

It is pronounced "pió". Look, the vocals "οι" together are pronounced "i". In Ancient Greek, it had another pronounciation (maybe oi) but in our days, it's much more simple: the vocals ι, η, υ and the compounds ει and οι have the same pronounciation: [i]
In the case that the diftong [oy] is to be pronounced, it takes an accent, like in the word κοροϊδεύω (to pull sb's leg). It's pronounced "koroidévo". The word "ρολόι": > "rolói".

A I forgot, it doen't take accent because it's considered monosyllabic and the only words that are excluded of this rule are πού and πώς:

Τα ερωτηματικά πού και πώς, είτε βρίσκονται σε ευθεία ερώτηση είτε σε πλάγια, π.χ., Πού πήγες; [ευθεία] Δε μας είπες πού πήγες. [πλάγια]

http://www.proz.com/kudoz/Greek...%8C%CF%82.html

Thank you very much! (yes, I remembered that "oi" is usually pronounced "i" only after I posted my first comment).

So, when it's "pio" - how do we know where to put the stress? There are actually two vowels here.

It's a good observation. I don't know.
The grammar was made for Greek speakers and as the word itself is toned on the last vowel and everybody knows it, it doen't take accent as there is a rule for monosyllabic words (like in Spanish: "dios"- pronounced on the last vowel). They forgot about the students of Greek who don't know how to pronounce it when reading it the first time. Okay, that's no real explanation. I really don't know :)

Thank you, this explanation is good enough for me :)