Aspect tells you why Greek makes an imperfective/perfective choice. This page tells you what you actually write. The mechanism is beautifully simple: every Greek verb keeps two stems in stock, and each tense and mood reaches for one of them. Learn a verb's two stems and you can build every form it has. Getting these two stems for each verb is, in practical terms, most of what "learning a Greek verb" means.
The two stems, and who uses them
| Stem | Built for | Feeds these forms |
|---|---|---|
| Imperfective (γράφ-) | ongoing / repeated / habitual | present, imperfect, continuous future (θα γράφω), imperfective subjunctive (να γράφω), imperfective imperative (γράφε) |
| Perfective (γράψ-) | a single complete act | aorist (έγραψα), simple future (θα γράψω), perfective subjunctive (να γράψω), perfective imperative (γράψε) |
The imperfective stem is the one you already know: it is simply the present-tense stem, the part before the ending in γράφ-ω, παίζ-ω, αγαπ-ώ. The perfective stem is the new thing to learn for each verb — and for the large regular classes, it is formed by a single, learnable operation.
Building the perfective stem: add -σ-
For most verbs, the perfective stem is the imperfective stem plus a -σ- (sigma), with a small, predictable sound change where the -σ- meets the final consonant of the stem.
| Verb (present) | Imperfective stem | Perfective stem | What happened |
|---|---|---|---|
| γράφω (write) | γράφ- | γράψ- | φ + σ → ψ |
| κόβω (cut) | κόβ- | κόψ- | β + σ → ψ |
| τρέχω (run) | τρέχ- | τρέξ- | χ + σ → ξ |
| ανοίγω (open) | ανοίγ- | ανοίξ- | γ + σ → ξ |
| διαβάζω (read) | διαβάζ- | διαβάσ- | ζ → σ |
| αγοράζω (buy) | αγοράζ- | αγοράσ- | ζ → σ |
| ακούω (hear) | ακού- | ακούσ- | vowel + σ (clean) |
| αγαπώ (love) | αγαπ- | αγαπήσ- | Class Β adds -ησ- |
| μιλάω (speak) | μιλ- | μιλήσ- | Class Β adds -ησ- |
Three patterns are worth pulling out:
- Labials (π, β, φ, and -εύω) meeting the -σ- fuse into ψ: γράφ- → γράψ-, κόβ- → κόψ-.
- Velars (κ, γ, χ) meeting the -σ- fuse into ξ: τρέχ- → τρέξ-, ανοίγ- → ανοίξ-.
- Class Β verbs almost all insert a vowel and take -ησ-: αγαπ- → αγαπήσ-, μιλ- → μιλήσ-, οδηγ- → οδηγήσ-.
This is a preview; the full set of merges, and the awkward cases, live on Forming the perfective stem and Perfective consonant changes. One honest wrinkle you should know now: the -ζω verbs split. Most go to -σ- (διαβάζω → διάβασα, αγοράζω → αγόρασα), but a sizeable group goes to -ξ- instead (παίζω → παίξ-, αλλάζω → άλλαξ-, φωνάζω → φώναξ-). There is no way to predict which from the present alone; you learn the perfective stem with the verb, exactly as you learn the past tense of an English irregular.
The two stems in action
Watch a single verb use both stems in one breath — imperfective for the ongoing frame, perfective for the bounded act inside it:
Διαβάζω εφημερίδα κάθε μέρα, αλλά αυτό το βιβλίο το διάβασα σε μία μέρα.
dhiavázo efimerídha káthe méra, alá aftó to vivlío to dhiávasa se mía méra
I read the newspaper every day, but this book I read in a single day.
Παίζω τένις τα σαββατοκύριακα, αλλά χθες έπαιξα για πρώτη φορά μπάσκετ.
pézo ténis ta savatokíriaka, alá chthes épeksa ya próti forá básket
I play tennis at weekends, but yesterday I played basketball for the first time.
And the perfective stem alone, doing its four jobs — aorist, simple future, subjunctive, imperative:
Άκουσα έναν περίεργο θόρυβο μέσα στη νύχτα.
ákusa énan períergho thórivo mésa sti níchta
I heard a strange noise in the middle of the night. (aorist)
Θα αγοράσω ψωμί στον δρόμο για το σπίτι.
tha aghoráso psomí ston dhrómo ya to spíti
I'll buy bread on the way home. (simple future)
Θέλω να σου μιλήσω για κάτι σοβαρό.
thélo na su milíso ya káti sovaró
I want to talk to you about something serious. (perfective subjunctive)
Τρέξε να προλάβεις το τρένο, φεύγει σε δύο λεπτά!
trékse na prolávis to tréno, févyi se dhío leptá!
Run and catch the train, it leaves in two minutes! (perfective imperative)
The perfective stem cannot stand alone
Here is a subtlety that catches a lot of learners. The perfective stem never surfaces as a bare present tense. γράψω on its own is not a Greek sentence — it does not mean "I write." The perfective stem is a dependent form: it only appears leaning on something — the particle θα (θα γράψω), the particle να (να γράψω), the augment that marks the past (έγραψα), or the imperative shape (γράψε). Greek grammarians literally call γράψω the εξαρτημένη — the "dependent."
Αύριο θα γράψω το γράμμα και θα το ταχυδρομήσω.
ávrio tha ghrápso to ghráma ke tha to tachidhromíso
Tomorrow I'll write the letter and post it.
So when you want an ongoing present ("I am writing"), you have no choice but the imperfective stem — because the perfective one is not allowed to head a present-tense clause at all. This is the deep reason the present has only one aspect.
What about irregular verbs?
Plenty of the commonest verbs do not build their perfective stem with a tidy -σ-; they suppletively swap in a different stem, the way English go/went does. βλέπω "see" has perfective δ- (θα δω, είδα); λέω "say" has perfective π- (θα πω, είπα); πηγαίνω "go" has πα- (θα πάω, πήγα); τρώω "eat" has φά- (θα φάω, έφαγα). These are learned individually and are gathered on the irregular verbs pages — but notice that the system is unchanged: they still have exactly two stems, one imperfective and one perfective, feeding the same forms. Irregularity is only in how the perfective stem is shaped, never in the two-stem architecture itself.
Δεν βλέπω καλά από δω· θα δω καλύτερα αν καθίσω μπροστά.
dhen vlépo kalá apó dho· tha dho kalítera an kathíso brostá
I can't see well from here — I'll see better if I sit at the front.
Common Mistakes
1. Using the perfective stem as a present tense. The present must be built on the imperfective stem; the perfective stem can't head a present.
❌ Κάθε μέρα διαβάσω την εφημερίδα με τον καφέ μου.
Incorrect — the present needs the imperfective stem: διαβάζω.
✅ Κάθε μέρα διαβάζω την εφημερίδα με τον καφέ μου.
káthe méra dhiavázo tin efimerídha me ton kafé mu
Every day I read the newspaper with my coffee.
2. Adding -σ- without the consonant merge. φ + σ becomes ψ; there is no ×γράφσω.
❌ Αύριο θα γράφσω δύο γράμματα.
Incorrect — φ + σ fuses to ψ: the perfective stem is γράψ-, so θα γράψω.
✅ Αύριο θα γράψω δύο γράμματα.
ávrio tha ghrápso dhío ghrámata
Tomorrow I'll write two letters.
3. Leaving the perfective stem bare, without θα or να. The dependent form cannot stand alone as a future.
❌ Αύριο γράψω το γράμμα οπωσδήποτε.
Incorrect — the perfective γράψω needs θα (or να): θα γράψω.
✅ Αύριο θα γράψω το γράμμα οπωσδήποτε.
ávrio tha ghrápso to ghráma opozdhípote
Tomorrow I'll write the letter for sure.
4. Building a Class Β perfective without the -ησ-. μιλάω forms μιλήσ-, not ×μιλάσ-.
❌ Θα μιλάσω με τον διευθυντή αύριο.
Incorrect — the Class Β perfective inserts -ησ-: θα μιλήσω.
✅ Θα μιλήσω με τον διευθυντή αύριο.
tha milíso me ton dhiefthindí ávrio
I'll speak with the manager tomorrow.
Key Takeaways
- Every verb has two stems: the imperfective (= the present stem, γράφ-) and the perfective (γράψ-).
- The imperfective stem feeds the present, imperfect, continuous future and imperfective subjunctive/imperative; the perfective stem feeds the aorist, simple future and perfective subjunctive/imperative.
- The perfective stem is usually the imperfective stem
- -σ-
- The perfective stem is dependent: γράψω never stands alone — it needs θα, να, the past augment, or the imperative.
- Irregular verbs still have exactly two stems (βλέπω/δ-, λέω/π-); only the shape of the perfective stem is unpredictable, never the architecture.
Now practice Greek
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Start learning Greek →Related Topics
- Building the Perfective StemB1 — The productive rule that manufactures the perfective stem from the present: Class Α adds -σ- to the stem (λύνω → λύσω), Class Β inserts a linking vowel first (αγαπώ → αγαπήσω), and the -εύω verbs give -εψ- (δουλεύω → δουλέψω). This one stem then feeds the aorist, the simple future, the perfective subjunctive and the perfective imperative.
- Perfective Consonant ChangesB1 — When the perfective -σ- lands on a consonant, the two fuse: labials (π, β, φ) give ψ, velars (κ, γ, χ) give ξ, and dentals plus most sibilants give plain σ. Learn the three merges by stem-final sound and you can read and build the aorist, the simple future and the perfective subjunctive of almost any verb — plus the honest -ζω split (παίζω → παίξω but διαβάζω → διάβασα).
- Aspect: The Heart of the Greek VerbA2 — Aspect — whether an action is seen as ongoing/repeated (imperfective) or as a single complete whole (perfective) — is the organizing spine of the Greek verb, and the hardest thing about it for English speakers. It forces a choice in the past, the future, the subjunctive and the imperative alike, shown here on one verb: γράφω.