Learn Hardcore Greek: Μιλάς πολύ γρήγορα, δεν καταλαβαίνω.

Greek drops subject pronouns because the verb ending shows the person. Μιλάς (ending -ς) already means you speak, and καταλαβαίνω (ending -ω) already means I understand. Pronouns (Εσύ, Εγώ) are added only for emphasis or contrast: Εσύ μιλάς πολύ γρήγορα, εγώ δεν καταλαβαίνω.

What’s the difference between μιλάς and λες?

μιλάω/μιλώ = to speak, talk (in general, a language, or to someone): Μιλάς ελληνικά;, Μιλάς σε μένα;
λέω = to say, tell (specific content): Τι λες;, Μου λες την ώρα; In this sentence, we’re talking about the manner of speaking, so μιλάς is right.

Is μιλάς from μιλάω or μιλώ? How do I conjugate it?

Both stems (μιλάω and the contracted μιλώ) are standard. Present tense:

εγώ μιλάω/μιλώ
εσύ μιλάς
αυτός/αυτή/αυτό μιλάει/μιλά
εμείς μιλάμε
εσείς μιλάτε
αυτοί/αυτές/αυτά μιλάνε/μιλούν(ε)

What does πολύ do here?

Πολύ intensifies the adverb γρήγορα, so it means very fast. Note:

Μιλάς πολύ γρήγορα = You speak very fast.
Μιλάς πολύ = You talk a lot (πολύ modifies the verb).
As an adjective, it’s πολύς/πολλή/πολύ = much/many: πολλοί άνθρωποι, πολλή δουλειά.

Why γρήγορα and not γρήγορος?

Γρήγορα is the adverb (how you speak). Γρήγορος/γρήγορη/γρήγορο is the adjective (describes a noun): ένα γρήγορο αυτοκίνητο. Many Greek adverbs come from adjectives by using the -α form: καλός → καλά, γρήγορος → γρήγορα.

Can I change the word order, like Πολύ γρήγορα μιλάς?

Yes. Greek allows flexible word order. Fronting Πολύ γρήγορα adds emphasis to “very fast.” The neutral, unmarked order is the original.

How do I say it more politely?

Formal/polite: Συγγνώμη, μιλάτε λίγο πιο αργά, παρακαλώ; Δεν σας καταλαβαίνω καλά.
Informal: Συγγνώμη, μίλα λίγο πιο αργά, σε παρακαλώ. Use μιλάτε/σας for formal or plural.

How do I say “I don’t understand you” specifically?

Add the object pronoun:

Informal singular: Δεν σε καταλαβαίνω.
Formal/plural: Δεν σας καταλαβαίνω. In simple tenses, the object pronoun comes before the verb.

Why δεν and not μην?

Use δεν to negate the indicative (statements/questions): Δεν καταλαβαίνω.
Use μην with the subjunctive/imperative: Μην μιλάς τόσο γρήγορα., Να μην ανησυχείς.

Pronunciation tips for this sentence?

Μιλάς [miˈlas] — stress on the last syllable.
Πολύ [poˈli] — υ sounds like [i].
Γρήγορα [ˈɣriɣoɾa] — γ is a voiced fricative [ɣ]; ρ is a tap/trill; stress on the first syllable.
Δεν [ðen] — δ is like English TH in this.
Καταλαβαίνω [kata.laˈveno] — αι = [e]; β = [v]; stress on -βαί-. Greek has one main stress mark per word (on one of the last three syllables). Accents are mandatory in writing.

Does the present tense mean both “speak” and “are speaking”?

Yes. Greek present covers both simple and progressive: μιλάς = “you speak/you are speaking,” δεν καταλαβαίνω = “I don’t understand/I’m not understanding.” Context clarifies.

How do I say “I didn’t understand”?

Δεν κατάλαβα. (aorist/past: a completed event)
Δεν σε κατάλαβα. (I didn’t understand you)
Δεν έχω καταλάβει. (I haven’t understood [yet], present perfect nuance)

Is the comma between the clauses correct?

Yes. Greek often uses a comma between two related independent clauses. A period would also be fine. Remember: the Greek question mark is ; and the semicolon/colon is the middle dot ·.

Can I shorten it to a quick request?

Yes: Πιο αργά, παρακαλώ. / Λίγο πιο αργά, παρακαλώ.
These are natural on-the-spot requests.

Any common learner mistakes with this sentence?

Using the adjective πολύς instead of the adverb πολύ before γρήγορα.
Saying λες instead of μιλάς for “speak.”
Mispronouncing δ ([ð]) and β ([v]).
Forgetting the accent marks (μιλάς, πολύ, γρήγορα, καταλαβαίνω).
Using δεν with an imperative instead of μην.
Putting the object pronoun in the wrong place (Δεν σε καταλαβαίνω, not Δεν καταλαβαίνω σε).