In short

Greek marks how sure you are and where the information came from with a small set of particles and frozen verbs. The centrepiece is θα used inferentially: θα φύγει is a prediction about the future, but θα έφυγε is a deduction about the past — “he must have left”. Around it sits a scale of adverbs, from οπωσδήποτε (“definitely”) down through μάλλον and ίσως to αποκλείεται (“no chance”), plus hearsay markers (λένε, δήθεν, τάχα, υποτίθεται ότι) that let you report a claim while refusing to vouch for it, and hedges (θα έλεγα, μου φαίνεται, κάπως) that soften your own.

The rule

1. θα is not a tense, it is a modal particle. With a future-oriented verb form it predicts; with a form that already points at the present or the past it infers. The verb form together with the time reference of the sentence decides which reading you get:

• θα + present → deduction about now: θα κοιμάται τώρα “she must be asleep now”. The same form is also the continuous future of lesson 24, so a forward-looking adverb restores the predictive reading: όλο το καλοκαίρι θα δουλεύω εκεί “I’ll be working there all summer”.
• θα + aorist → deduction about a completed past event: θα έφυγε “he must have left”.
• θα + perfect → deduction about a past event with present relevance: θα το έχει ξεχάσει “he must have forgotten it”.
• θα + imperfect → deduction about a past state or ongoing situation: θα ήταν δέκα η ώρα “it must have been ten o’clock”.

Contrast Θα φύγει αύριο (perfective non-past = future, “he will leave”) with Θα έφυγε χθες (aorist = inference, “he must have left yesterday”). Same particle, opposite time reference. This is the single most useful thing in the lesson, and the thing English speakers most often mis-parse.

2. μπορεί / πιθανόν / ίσως / ενδεχομένως take να + either stem. Here the aspect choice is the ordinary one (μπορεί να φεύγει “may be leaving” vs μπορεί να φύγει “may leave”), and a past stem after να gives a past possibility (μπορεί να έφυγε). Epistemic μπορεί is impersonal: it is frozen in the 3rd person singular and never agrees with the subject. Person-marked μπορώ/μπορείς/μπορούν is the ability verb, a different word in practice.

3. πρέπει να is ambiguous, and aspect disambiguates it. With a perfective non-past it is normally deontic (obligation): πρέπει να φύγει “he has to leave”. With a past form it can only be epistemic (inference): πρέπει να έφυγε “he must have left”. Like μπορεί, epistemic πρέπει is impersonal.

4. Adverbs of certainty attach to a plain indicative (σίγουρα ήρθε, μάλλον δεν το άκουσε) or to θα (μάλλον θα ήρθε, δύσκολα θα προλάβουμε). Only ίσως, πιθανόν and ενδεχομένως pull the verb into να / the bare subjunctive — and for a plain guess ίσως does not take θα: not Ίσως θα έρθει, but Ίσως έρθει. The one place ίσως θα is at home is the polite conditional: Ίσως θα ήταν καλύτερα να φύγουμε.

5. Hearsay markers strip you of responsibility. λέει / λένε (“they say”), υποτίθεται ότι (“supposedly”), κατά τα λεγόμενα (“according to what X says”) are neutral reports; δήθεν and τάχα add open scepticism — the speaker signals that the claim is probably false.

6. Negation after να is μην, not δεν: μπορεί να μην ήρθε.

Forms & patterns

θα + present — inference about the present (δουλεύω)

θα + αόριστος — inference about a completed past event (φεύγω). Note the augment and the stress shift: έφυγα → φύγαμε.

θα + παρακείμενος — inference with present relevance (ξεχνάω)

θα + παρατατικός — inference about a past state (είμαι)

Impersonal μπορεί / πρέπει with both stems (φεύγω)

The certainty scale

Hearsay and hedges

Examples

Common mistakes

• Ίσως θα έρθει. -> Ίσως έρθει. / Ίσως να έρθει. — for a guess about what will happen ίσως does not take θα; it takes the bare perfective non-past or να. (The polite Ίσως θα ήταν καλύτερα… is the conditional, a different pattern.)
• Μπορούν να έφυγαν. -> Μπορεί να έφυγαν. — epistemic μπορεί is impersonal and frozen in the 3rd singular; the person is carried by the verb after να.
• Θα έφυγε αύριο. -> Θα φύγει αύριο. — θα + aorist is an inference about the past, so it cannot host a future adverb; the future needs the perfective non-past.
• Πιθανόν θα το ξέχασε. -> Πιθανόν να το ξέχασε. — πιθανόν and ενδεχομένως pattern like ίσως: να, not θα.
• Μπορεί να δεν ήρθε. -> Μπορεί να μην ήρθε. — inside a να-clause the negator is μην; δεν only negates an indicative.
• Πρέπει να φύγει χθες. -> Πρέπει να έφυγε χθες. — with a perfective non-past πρέπει να looks forwards, not back; the deduction about the past needs a past form.
• Μάλλον να ήρθε. -> Μάλλον ήρθε. / Μάλλον θα ήρθε. — μάλλον, προφανώς and σίγουρα modify an indicative (or θα + verb) and never introduce να.
• Υποτίθεται να έρθει στις οκτώ. -> Υποτίθεται ότι θα έρθει στις οκτώ. — υποτίθεται takes ότι/πως plus a full clause, not να.
• Κατά τα λεγόμενα του, έφυγε. -> Κατά τα λεγόμενά του, έφυγε. — λεγόμενα is stressed on the third syllable from the end, so the following clitic forces a second accent.

Tips

• Test any θα you meet by asking “could this be tomorrow?”. If the verb form points backwards or at right now, θα means must, not will.
• The three-way drill θα φύγει / θα φεύγει / θα έφυγε (“he’ll leave” / “he must be leaving” / “he must have left”) is worth saying aloud until it is automatic; it is the fastest way to internalise the whole system.
• Remember the two frozen impersonals, μπορεί and πρέπει. If you find yourself conjugating them for a guess, you have slipped back into ability or obligation.
• μπορεί and αποκλείεται always want να; ίσως, πιθανόν and ενδεχομένως take να or a bare verb (Ίσως έρθει, Ενδεχομένως άλλαξε γνώμη); μάλλον, προφανώς, σίγουρα, οπωσδήποτε, δύσκολα all want an indicative or θα. Learn the three lists as lists.
• δήθεν and τάχα are not neutral. Using them about a colleague’s explanation says out loud that you don’t believe it — reach for υποτίθεται ότι or κατά τα λεγόμενά του when you only mean to cite a source.
• Greek hedges cluster at the front of the sentence: Θα έλεγα ότι…, Μου φαίνεται πως…, Απ’ ό,τι καταλαβαίνω… Starting there buys you thinking time and sounds far more idiomatic than a tacked-on ίσως at the end.
• In writing, κατά πάσα πιθανότητα and ενδεχομένως carry the formal register; in speech μάλλον and μπορεί να do almost all the work.