ξέρω (to know) | Greek Grammar

Of the verbs you use every single day, ξέρω is the one with the fewest forms. It has a present, an imperfect and whatever you can build out of those two — and that is the whole verb. (It is not alone in this: ανήκω "belong" and αξίζω "be worth" are defective the same way, as the irregular verbs overview notes. ξέρω is simply the one you need hourly.)

Everything ξέρω has

Person	Present	Imperfect
εγώ	ξέρω	ήξερα
εσύ	ξέρεις	ήξερες
αυτός / αυτή	ξέρει	ήξερε
εμείς	ξέρουμε	ξέραμε
εσείς	ξέρετε	ξέρατε
αυτοί	ξέρουν	ήξεραν — ξέρανε (informal)

Everything else this verb can do is those two stems with a particle in front:

Slot	Form	Gloss
Future	θα ξέρω	I will know, from then on
After να	να ξέρω	the present stem, nothing else available
Conditional	θα ήξερα	I would know, if…
Command slot	να ξέρεις / να ξέρετε	just so you know…
Gerund	ξέροντας (formal)	knowing — mostly in writing

Everything ξέρω does not have

Stated in those words, because a conjugation app will happily invent them for you:

Aorist — does not exist. There is no ξέρησα, no έξερα as a past tense. The past of ξέρω is the imperfect ήξερα, full stop.
Perfective (dependent) stem — does not exist. Every other verb offers a choice after θα and να — θα γράφω against θα γράψω. ξέρω offers one form, so θα ξέρω and να ξέρω carry no aspect contrast at all; there is nothing to choose between.
Perfect and pluperfect — do not exist. With no perfective stem there is no -ει form to put after έχω or είχα, which is why "if I had known" comes out as αν το ήξερα. That knock-on effect is worked through on the past conditional.
Imperative — does not exist. The slot is filled by να ξέρεις / να ξέρετε.
-μένος participle — does not exist.
Mediopassive — no paradigm. The one live use is reciprocal and plural: ξερόμαστε, ξέρεστε, ξέρονται "know each other" (informal), where careful speech prefers γνωριζόμαστε.

💡

The gaps are not an accident of history you have to work around — they are a meaning. ξέρω names a state, and a state has no natural endpoint to package up perfectively. When a Greek sentence needs the endpoint — the moment the knowledge arrives — the language switches verbs rather than bending this one.

What fills the holes

You want to say	Use	Why
I found out, I'll find out	έμαθα, θα μάθω, να μάθω	μαθαίνω supplies the bounded event
I met (a person)	γνώρισα	the first-acquaintance event
I have known / had known	ξέρω / ήξερα + a time phrase	no perfect exists to build
formal register: know, be aware	γνωρίζω(formal)	full paradigm: γνώρισα, να γνωρίσω, έχω γνωρίσει

Θα το μάθουμε σίγουρα μέχρι την Παρασκευή.

tha to máthume síghura méchri tin Paraskeví

We'll know for sure by Friday.

Τον γνώρισα πέρσι σε ένα συνέδριο στη Θεσσαλονίκη.

ton ghnórisa pérsi se éna sinédhrio sti Thesaloníki

I met him last year at a conference in Thessaloniki.

Δεν γνωρίζω αν έχει εκδοθεί ακόμα η απόφαση.

dhen ghnorízo an échi ekdhothí akóma i apófasi

I am not aware whether the decision has been issued yet. (formal — in speech among friends this sounds stiff, and ξέρω is what you want)

Which of the two to reach for in a given sentence is the subject of ξέρω vs γνωρίζω.

Government

Frame	Example	Reading
accusative — people and things	Τον ξέρω. / Ξέρω ελληνικά.	be acquainted with, have a command of
ξέρω να + verb	Ξέρω να κολυμπάω.	a learned skill, not ability today
ξέρω ότι / πως + clause	Ξέρω ότι άργησε.	a fact
embedded question: αν, τι, πού, ποιος	Ξέρεις πού μένει;	knowing the answer to something
ξέρω από accusative	Ξέρει από κρασιά.	be a good judge of, know one's way around

Ξέρεις κανένα καλό συνεργείο εδώ κοντά;

kséris kanéna kaló sineryío edhó kondá;

Do you know a decent garage around here?

Ρώτα τον Θανάση, αυτός ξέρει από μηχανές.

róta ton Thanási, aftós kséri apó michanés

Ask Thanasis, he's the one who knows about engines.

Δεν ήξερα ότι μαγειρεύεις τόσο καλά.

dhen íksera óti mayirévis tóso kalá

I had no idea you cooked this well.

The skill frame ξέρω να is the one that collides with English can: "I can swim" is ξέρω να κολυμπάω (I learned how), while "I can't swim today, my ear hurts" is μπορώ. The split is drawn on ξέρω να vs μπορώ να.

Set phrases

Phrase	Meaning
ξέρω 'γω (informal)	dunno, search me
πού να ξέρω	how am I supposed to know?
ξέρεις κάτι;	you know what? — opens a change of tack
όπως ξέρεις	as you know
ποιος ξέρει	who knows, maybe
ξέρω τι λέω	I know what I'm talking about
δεν ξέρω τι μου γίνεται	I'm all over the place (informal)
απ' όσο ξέρω	as far as I know

Απ' όσο ξέρω, το μαγαζί κλείνει στις οκτώ.

ap' óso kséro, to maghazí klíni stis októ

As far as I know, the shop closes at eight.

Να ξέρεις ότι είμαι εδώ, ό,τι κι αν γίνει.

na kséris óti íme edhó, óti ki an yíni

Just so you know, I'm here, whatever happens.

Ξερόμαστε χρόνια, από τον στρατό.

kserómaste chrónia, apó ton strató

We've known each other for years, since our army days. (informal)

Common Mistakes

Manufacturing a past event out of ξέρω. Learners who need "I found out" build a regular aorist from the present stem. There is no such form, and the sentence has to change verbs.

❌ Χθες ξέρησα ότι παντρεύεται η Ελένη.

Incorrect — ξέρω has no aorist. The bounded 'found out' is έμαθα.

✅ Χθες έμαθα ότι παντρεύεται η Ελένη.

chthes ématha óti pandrévete i Eléni

Yesterday I found out Eleni is getting married.

The same repair works forwards: for "I'll know tomorrow" in the sense of finding out, Greek says Θα το μάθω αύριο. Plain θα ξέρω is good Greek too — it just says you will be in a state of knowing, not that the news will reach you.

Key Takeaways

The entire verb: ξέρω · ήξερα · θα ξέρω, plus να ξέρω and θα ήξερα built from the same two stems.
The past is an imperfect with ή-: ήξερα, dropping to ξέραμε / ξέρατε where the stress moves off the augment.
No aorist, no perfective stem, no perfect, no imperative, no -μένος participle — and consequently no aspect choice after θα or να.
The holes are filled by other verbs: μαθαίνω (έμαθα, θα μάθω) for finding out, γνωρίζω for meeting someone and for the formal register.
Frames to keep straight: accusative for people and things, ξέρω να for a learned skill, ξέρω από for being a good judge of something.

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