Obligation: πρέπει / έπρεπε | Greek Grammar

The frame πρέπει να and its past έπρεπε να are built on the impersonal modal page, which owns the machinery: πρέπει never conjugates, the person rides on the να-verb, and δεν πρέπει να prohibits. Assume all of that. This page is about two things that page leaves on the table, and they are the two that actually trip people up in conversation: πρέπει used to deduce rather than to oblige, and the way the aspect of the verb after να quietly decides what kind of obligation you have just handed someone.

One word, two pressures

English speakers already have this split and never notice it. You must wash your hands is a duty. You must be exhausted is a deduction — nobody is ordering you to be exhausted. English uses the same word must for both, and so does Greek with πρέπει να. The technical names are deontic (duty, from the outside world) and epistemic or evidential (the speaker's inference, from the evidence in front of them).

What makes the Greek version worth a page of its own is that the two readings are separated by a signal English does not have: what you put after να — its tense, its aspect, and whether the state it names is one anybody can control. Get that right and the ambiguity mostly evaporates.

The evidential πρέπει: "he must be…"

Point πρέπει να at a state — being tired, knowing something, being at home — and it stops obliging and starts deducing. You are reading evidence and reporting your best conclusion.

Πρέπει να είναι κουρασμένος, δούλευε όλο το σαββατοκύριακο.

prépi na íne kurazménos, dhúleve ólo to savatokíriako

He must be tired — he worked the whole weekend.

Πρέπει να ξέρει κάτι που δεν μας λέει.

prépi na kséri káti pu dhen mas léi

He must know something he isn't telling us.

Δεν σηκώνει το τηλέφωνο· πρέπει να είναι ακόμα στη δουλειά.

dhen sikóni to tiléfono· prépi na íne akóma sti dhuliá

She's not picking up the phone; she must still be at work.

Notice what these have in common: nobody can decide to be tired, to know something, or to be at work at this instant. When the να-verb names something the subject cannot switch on at will, the obligation reading has almost nothing to work with, and the deduction is what the listener takes. Almost, rather than nothing: a state can be a precondition for something else, and then the requirement reading comes back — Το παιδί πρέπει να είναι κουρασμένο για να κοιμηθεί ("the child has to be tired in order to sleep") is a requirement, not a guess. Controllability is what decides, and a για να-clause is the usual sign that a requirement is meant.

A past tense after να tips it to deduction

Here is the cleanest signal in the system. Put a genuinely past verb after να — an aorist, an imperfect — and the sentence is heard as an inference: a requirement cannot reach back and be imposed on an event that is already over, so what is left is a guess.

Πρέπει να έφυγε νωρίς, το φως στο γραφείο του είναι σβηστό.

prépi na éfiye norís, to fos sto ghrafío tu íne zvistó

He must have left early — the light in his office is off.

Πρέπει να το ξέχασε τελείως, δεν το ανέφερε καθόλου.

prépi na to kséchase telíos, dhen to anéfere kathólu

He must have completely forgotten it — he didn't mention it at all.

Πρέπει να κοιμόταν όταν χτύπησα το κουδούνι.

prépi na kimótan ótan chtípisa to kudhúni

She must have been asleep when I rang the bell.

The aspect keeps working inside the deduction: πρέπει να έφυγε (aorist) guesses at a completed event, πρέπει να κοιμόταν (imperfect) guesses at a state that was in progress.

One important exception keeps this from being a blanket rule about "past-looking" complements. The perfect subjunctive (να έχει φύγει) has a present auxiliary, and it stays open to both readings — which is exactly how Greek expresses a deadline:

Πρέπει να έχεις τελειώσει την αναφορά μέχρι τις πέντε.

prépi na échis teliósi tin anaforá méchri tis pénde

You have to have the report finished by five. (obligation)

Πρέπει να έχει φτάσει ήδη, ξεκίνησε πριν από δύο ώρες.

prépi na échi ftási ídhi, ksekínise prin apó dhío óres

He must have arrived already — he set off two hours ago. (deduction)

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Greek gives you a tense contrast where English gives you nothing. "He must have left" is one English shape doing two jobs; Greek splits it — πρέπει να έφυγε (past tense inside) is a guess about what happened, πρέπει να έχει φύγει μέχρι τις πέντε (perfect inside, plus a deadline) is a requirement about what must be true by then.

Negating a deduction

Negation is where the two readings stop being tidy, and it is worth being honest about it. Δεν πρέπει να… is read first as a prohibition ("mustn't"), which is the sense the impersonal page documents. In relaxed speech it also does duty as a negated guess ("can't be, surely isn't"), and only the situation tells you which.

If you want the deduction to be unmistakable, put the negation inside the να-clause with μην, or switch to δεν μπορεί να for a flat denial:

Πρέπει να μην κοιμήθηκε καθόλου, έχει μαύρους κύκλους.

prépi na min kimíthike kathólu, échi mávrus kíklus

He must not have slept at all — he's got rings under his eyes.

Δεν πρέπει να το μάθει κανείς πριν το ανακοινώσουμε.

dhen prépi na to máthi kanís prin to anakinósume

Nobody must find out before we announce it.

θα πρέπει να — the hedged version

Adding θα in front gives θα πρέπει να, which softens whichever reading is in play: a slightly less blunt obligation, or a slightly more tentative deduction. It is completely standard (neutral) and extremely common in news writing and careful speech.

Θα πρέπει να έχει ξεχάσει το ραντεβού μας, δεν του μοιάζει καθόλου.

tha prépi na échi ksechási to randevú mas, dhen tu miázi kathólu

He must have forgotten our appointment — it's really not like him.

For the other everyday way of guessing — bare θα in front of the verb, with no πρέπει at all (Θα κοιμάται, "he's probably asleep") — see θα for probability. Rough guide: θα is the quick reflexive guess, πρέπει να is the guess you have reasoned your way to.

Aspect decides what kind of obligation

Now the second half of the page. Greek forces you to choose a stem after να — perfective or imperfective — and English "must" gives you no clue which. With πρέπει the choice carries a specific meaning: a single instruction versus a standing rule.

Πρέπει να πάρεις το φάρμακό σου τώρα, πέρασε η ώρα.

prépi na páris to fármakó su tóra, pérase i óra

You need to take your medicine now — it's past the time.

Πρέπει να παίρνεις το φάρμακό σου κάθε βράδυ, όχι μόνο όταν πονάς.

prépi na pérnis to fármakó su káthe vrádhi, óchi móno ótan ponás

You're supposed to take your medicine every evening, not just when you're in pain.

The English translations drift apart on their own: the perfective comes out as "do this now", the imperfective as "you're supposed to / you should be doing this". That second one is the sound of a policy, a habit, a piece of standing advice — which is why almost every proverb-style piece of Greek moralising uses the imperfective.

The same split runs through the prohibition:

Δεν πρέπει να λες ψέματα στα παιδιά, το καταλαβαίνουν.

dhen prépi na les psémata sta pedhiá, to katalavénun

You shouldn't lie to children — they can tell.

Δεν πρέπει να του πεις τίποτα απόψε, είναι ήδη αναστατωμένος.

dhen prépi na tu pis típota apópse, íne ídhi anastatoménos

You mustn't say anything to him tonight — he's already upset.

The first bans a practice for life; the second bans one act tonight. Same modal, same negation — the whole difference is λες against πεις.

Έπρεπε: "had to" and "should have" in one form

The past έπρεπε να is genuinely two-faced, and learners are often taught only half of it. With an ordinary dependent verb after να, the sentence is open between an obligation that was carried out and one that was not:

Έπρεπε να φύγω στις έξι, αλλά με κράτησε ο διευθυντής.

éprepe na fígho stis éksi, alá me krátise o dhiefthindís

I had to leave at six, but the manager kept me back.

Depending on what follows, that same Έπρεπε να φύγω στις έξι is either "I had to leave at six (and I did)" or "I should have left at six (and didn't)". Greek does not disambiguate it for you — the continuation does. When you want the regret to be unmistakable, put a pluperfect inside the clause:

Έπρεπε να είχα φύγει στις έξι, τώρα θα κολλήσω στην κίνηση.

éprepe na ícha fíyi stis éksi, tóra tha kolíso stin kínisi

I should have left at six — now I'll get stuck in traffic.

The heavier, more layered forms of regret — θα έπρεπε να είχα…, μακάρι να μην…, ας μην είχα… — are gathered on counterfactual and regret.

Common Mistakes

1. Reaching for the perfective when you mean a standing rule. Both are grammatical; the aspect decides whether you have handed someone one instruction or laid down a policy.

Πρέπει να πλένεις τα χέρια σου πριν φας.

prépi na plénis ta chéria su prin fas

You should wash your hands before you eat. (imperfective — a standing rule)

Πρέπει να πλύνεις τα χέρια σου, ήρθες μόλις από τον κήπο.

prépi na plínis ta chéria su, írthes mólis apó ton kípo

Wash your hands — you've just come in from the garden. (perfective — this once)

2. Reaching for a past verb when you mean a future requirement. Both of these are good Greek, but the tense inside the clause changes the sentence from a deduction into a duty.

Πρέπει να έφυγε ήδη, δεν βλέπω την τσάντα του.

prépi na éfiye ídhi, dhen vlépo tin tsánda tu

He must have left already — I can't see his bag. (a deduction about the past)

Πρέπει να φύγει μέχρι τις πέντε, αλλιώς θα χάσει την πτήση.

prépi na fíyi méchri tis pénde, aliós tha chási tin ptísi

He has to leave by five, otherwise he'll miss his flight. (a requirement)

3. Negating a guess with δεν and getting a rule. Both are correct; they say different things, and the one you want for "he can't possibly know" is not the δεν πρέπει one.

Δεν πρέπει να το ξέρει κανείς άλλος στο γραφείο.

dhen prépi na to kséri kanís álos sto ghrafío

Nobody else in the office is to know about it. (a prohibition — the default reading)

Δεν μπορεί να το ξέρει, μόλις το αποφασίσαμε.

dhen borí na to kséri, mólis to apofasísame

There's no way he knows — we only just decided. (a flat denial of the guess)

4. Expecting Έπρεπε να to mean "should have" on its own. It carries both readings; if the point is that it did not happen, spell it out with the pluperfect.

Έπρεπε να δουλέψω και την Κυριακή, δεν προλάβαινα αλλιώς.

éprepe na dhulépso ke tin Kiriakí, dhen prolávena aliós

I had to work on Sunday too — I wasn't going to finish otherwise. (and I did)

Έπρεπε να σε είχα ακούσει τότε που μου το έλεγες.

éprepe na se ícha akúsi tóte pu mu to éleyes

I should have listened to you back when you were telling me. (and I didn't)

Key Takeaways

πρέπει να obliges and deduces: Πρέπει να πλύνεις τα χέρια σου is a duty, Πρέπει να είναι κουρασμένος is an inference — exactly the double life of English must.
The deduction is the default reading when the να-verb names a state nobody can switch on at will (είναι, ξέρει, κοιμάται) — unless that state is a precondition for something else.
A real past tense after να (έφυγε, κοιμόταν) steers the sentence to the inference reading; the perfect subjunctive (να έχεις τελειώσει) stays open, which is how Greek states a deadline.
For an unmistakable negated guess, negate inside the clause (πρέπει να μην…) or use δεν μπορεί να…; bare δεν πρέπει να is heard first as a prohibition.
Aspect sets the kind of obligation: perfective = one instruction now (Πρέπει να πάρεις), imperfective = a standing rule or "you're supposed to" (Πρέπει να παίρνεις).
Έπρεπε να is open between "had to (and did)" and "should have (and didn't)"; a pluperfect inside (Έπρεπε να είχα φύγει) makes it regret.

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