Aspect tells you why Greek makes an imperfective/perfective choice. This page shows how that contrast becomes morphology in the productive core: most verbs keep an imperfective and a perfective stem in stock, and the central tense and mood forms select between them. One-stem and strongly suppletive verbs need extra principal-part information, covered below.
The two stems, and who uses them
| Stem | Built for | Feeds these forms |
|---|---|---|
| Imperfective (γράφ-) | internal / unbounded / open habit | present, imperfect, continuous future (θα γράφω), imperfective subjunctive (να γράφω), imperfective imperative (γράφε) |
| Perfective (γράψ-) | a bounded occurrence or finite series | aorist (έγραψα), simple future (θα γράψω), perfective subjunctive (να γράψω), perfective imperative (γράψε) |
The imperfective stem is the one you already know: it is the present-tense stem, the part before the ending in γράφ-ω, παίζ-ω, αγαπ-ώ. For the large regular classes, the perfective is formed by a learnable operation. These labels describe the speaker's viewpoint, not objective completion or event count: one event can be viewed internally, and a delimited repeated series can be bounded.
Building the perfective stem: add -σ-
For the large regular sigma-forming classes, the perfective stem is the imperfective stem plus -σ-, with a predictable sound change where sigma meets the final consonant.
| 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-.
- Many Class Β verbs insert -ησ-: αγαπ- → αγαπήσ-, μιλ- → μιλήσ-, οδηγ- → οδηγήσ-. Smaller productive groups use -ασ- or -εσ-.
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 bare perfective non-past cannot stand alone
Here is a subtlety that catches a lot of learners. γράψω on its own is not an independent present or future clause. This person-marked perfective non-past is the dependent form (εξαρτημένη): it normally follows a particle such as θα (θα γράψω) or να (να γράψω). The same perfective series also supplies independent aorist and imperative forms, έγραψα and γράψε, with their own morphology.
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 a whole perfective series with tidy -σ-. Their principal parts are suppletive: βλέπω → θα δω / είδα, λέω → θα πω / είπα, πηγαίνω → θα πάω / πήγα, τρώω → θα φάω / έφαγα. They still express the aspect contrast, but their dependent perfective and aorist may not reduce to one reusable string. At the other edge, one-stem verbs such as περιμένω have no separate aspect form at all, so context carries both readings. Learn these from the irregular verbs and their reference cards rather than forcing a universal two-stem template.
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
- Most productive verbs contrast an imperfective present stem (γράφ-) with a perfective stem (γράψ-); suppletive and one-stem verbs need separate principal-part notes.
- 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 non-past γράψω is dependent and normally follows θα or να; the aorist and imperative use the same perfective series with their own morphology.
- Stem choice marks internal/open versus bounded viewpoint; it does not mechanically count events or assert objective completion.
Related Topics
- Building the Perfective StemB1 — The productive regular derivation of the perfective stem: Class Α adds -σ- (λύνω → λύσω), Class Β uses a linking vowel (αγαπώ → αγαπήσω), and -εύω gives -εψ- (δουλεύω → δουλέψω).
- 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 organizes the Greek verb around two viewpoints: an internal process or open habit (imperfective), versus one occurrence or a finite series packaged as bounded (perfective), illustrated across the system with γράφω.