ξέρω (to know)

Among the most frequent verbs, ξέρω has an unusually small paradigm: a present stem, the imperfect ήξερα, and no perfective stem. ανήκω "belong" and αξίζω "be worth" are similarly defective, as the irregular verbs overview notes.

Everything ξέρω has

Outside the indicative, the same present and imperfective material supplies these forms; no second aspect stem appears:

The stress alternation in ήξερα ~ ξέραμε is explained with the internal augment.

Everything ξέρω does not have

These are lexical gaps, not forms to build by analogy:

• Aorist — does not exist. Standard Greek has no ξέρησα; the past form is the imperfect ήξερα.
• Perfective (dependent) stem — does not exist. Verbs with a productive aspect pair contrast forms such as θα γράφω and θα γράψω. ξέρω is instead a one-stem defective verb: θα ξέρω and να ξέρω have no perfective counterpart and therefore make no aspect choice.
• 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.

What fills the holes

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

Government

The skill frame ξέρω να can overlap English can: ξέρω να κολυμπάω means I learned how, while "I can't swim today; my ear hurts" is δεν μπορώ να κολυμπήσω. See ξέρω να vs μπορώ να.

Set phrases

Common Mistakes

Manufacturing a past event out of ξέρω. For "I found out," do not build an aorist from ξέρω; switch to μαθαίνω.

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 core finite paradigm is built from ξέρω and imperfect ήξερα: θα ξέρω, να ξέρω and θα ήξερα reuse those same materials.
• 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 is neutral/current for acquaintance and formal-leaning for informational knowledge.
• Frames to keep straight: accusative for people and things, ξέρω να for a learned skill, ξέρω από for being a good judge of something.