In short

A temporal clause says when something happens: όταν “when”, μόλις “as soon as”, πριν “before”, ενώ “while”, μέχρι να “until”. In English the tricky part is the tense (“when you arrive”, not “when you will arrive”). In Greek the tricky part is the stem: each conjunction forces either the continuous stem (έρχεται, φεύγει, τελειώνει) or the simple/perfective stem (έρθει, φύγει, τελειώσει), and the choice is not free — it follows from whether you mean one completed event or a repeated/ongoing situation.

Two rules carry most of the work. First: after όταν, μόλις, αφού referring to a single future event, use the bare perfective non-past — the form you already know from να and θα, but with no θα and no να: Όταν τελειώσω, θα φύγω. Second: πριν (να), μέχρι να, ώσπου να always take that same dependent form, whatever the time frame — you can never say πριν έφυγα.

The rule

1. The two stems, one more time

Every Greek verb has a continuous stem (φεύγ-, έρχ-, τελειών-) and a simple/perfective stem (φύγ-, έρθ-, τελειώσ-). The perfective stem cannot stand alone as a tense: it needs θα, να, an imperative, or — and this is the point of this lesson — a temporal conjunction. Grammars call this bare perfective non-past the εξαρτημένη (“dependent”), because it always depends on something in front of it.

2. Conjunctions by the stem they force

Read the table as two blocks. όταν / μόλις / αφού are flexible: they take whichever stem matches your meaning. πριν / μέχρι να / ώσπου να are rigid: the dependent, always, even when the main clause is in the past.

3. One event vs. a habit

Όταν is the clearest test case, because it takes both stems with a real difference in meaning:

The main clause follows suit: a habit is stated in the present (πίνουμε), a single future event with θα (θα πιούμε). To make the habitual reading explicit, use κάθε φορά που instead of όταν — it is always continuous. όποτε is habitual with the continuous (Όποτε πονάει, παίρνει ένα χάπι), but it keeps the dependent when you leave the occasion open: Όποτε με χρειαστείς, θα σε βοηθήσω.

4. Simultaneity: ενώ, καθώς, την ώρα που

These three set two situations side by side, so the verb is continuous: an imperfect (έτρωγα, διάβαζε) for the past, a present for now. A dependent after ενώ is simply ungrammatical, and so is να. For a future backdrop the continuous simply keeps its θα: Ενώ θα ετοιμάζεσαι, θα διαβάζω. And καθώς has a second, momentary sense (“the moment that”), where it takes an aorist: Καθώς τον είδα, κατάλαβα.

Ενώ έτρωγα, χτύπησε το τηλέφωνο. — the eating is the background, the ringing the event.

5. πριν + clause vs. πριν από + accusative

πριν in front of a verb introduces a clause and takes the dependent; the optional να changes nothing: πριν φύγεις = πριν να φύγεις. πριν από in front of a noun or phrase is a preposition and takes the accusative: πριν από το μάθημα, πριν από δύο μέρες. Before a noun the από is optional — πριν από δύο μέρες and πριν δύο μέρες are both standard — but never πριν από before a verb.

The same split works for μετά: μετά το μάθημα (preposition + accusative) vs. αφού τελειώσει το μάθημα (conjunction + dependent).

Forms & patterns

The dependent of the three verbs this lesson builds on

All six persons; note that these are the very forms you already use after θα and να.

The same verbs in the continuous non-past (for ενώ, καθώς, habitual όποτε)

The imperfect (for ενώ, καθώς, την ώρα που in the past)

Sentence frames

Note that όταν, μόλις and αφού also work in the past, and there they take a past tense, not the dependent: Μόλις έφτασε, μας τηλεφώνησε. Όταν έφτασαν, ήταν πολύ αργά. The dependent rule is about the non-past.

Examples

Common mistakes

• Όταν θα έρθεις, θα φάμε. -> Όταν έρθεις, θα φάμε. Greek, like English, drops the future marker after όταν, μόλις and αφού: the θα belongs to the main clause.
• Όταν θα τελειώνω… -> Όταν τελειώσω… Two errors at once: no θα after όταν, and a single future event needs the dependent, not the continuous.
• Πριν έφυγα, έκλεισα τα φώτα. -> Πριν φύγω, έκλεισα τα φώτα. πριν never takes a past tense: the dependent φύγω covers past and future alike, and the main clause carries the tense.
• Πριν από φύγεις -> Πριν φύγεις. πριν από is a preposition and only takes a noun phrase in the accusative: πριν από το ταξίδι.
• Ενώ να τρώω, μιλάει. -> Ενώ τρώω, μιλάει. ενώ, καθώς and την ώρα που take a plain indicative — never να, and never the dependent.
• Ενώ έφαγα, χτύπησε το τηλέφωνο. -> Ενώ έτρωγα, χτύπησε το τηλέφωνο. Simultaneity needs the imperfect (background), not the aorist (completed point).
• Μέχρι έρθεις -> Μέχρι να έρθεις. With a verb, μέχρι needs να (or μέχρι που with a past tense: Περίμεναν, μέχρι που χτύπησε το κουδούνι); bare μέχρι introduces a noun: μέχρι τις πέντε.
• Την ώρα πού έφυγε ήταν οκτώ. -> Την ώρα που έφυγε ήταν οκτώ. που here is the unaccented relativiser; the accented πού is only the question word “where?”.
• Μόλις θα φτάσω, θα σε πάρω. -> Μόλις φτάσω, θα σε πάρω. μόλις behaves exactly like όταν: dependent inside, θα outside.

Tips

• One test decides the stem after όταν/μόλις/αφού: once or every time? Once → dependent (έρθει). Every time → continuous (έρχεται).
• Learn πριν, μέχρι να and ώσπου να as a fixed block: they take the dependent and nothing else, so you never have to think about tense inside them.
• If you can put “will” in the English but not in the Greek, you have found the θα rule: “when you will arrive” is normal English thinking, but Όταν θα έρθεις is wrong.
• Την ώρα που means “at the moment when”, not “the hour that” — even though it looks like a noun plus a relative clause. Keep που unaccented; πού with an accent asks “where?”.
• ενώ has a second life as “whereas” (Εγώ δουλεύω, ενώ αυτός κοιμάται). The continuous stem is obligatory only in the temporal sense; the contrastive ενώ takes whatever tense the contrast needs, the aorist included.
• After a third-syllable-stressed word a following weak pronoun pulls in a second accent: μόλις τελειώσει το μάθημά μας, φεύγουμε.
• Word order is free: Θα φύγω όταν τελειώσω is as good as Όταν τελειώσω, θα φύγω. Only the fronted version takes a comma.