In short

Greek has two ordinary past tenses and you must choose between them in every past sentence. The aorist presents a past action as one bounded whole: it happened, it finished, the sentence draws a line around it. The imperfect presents it as unbounded: a background, a description, a habit, or something still in progress at the moment you are talking about. There is no new morphology here - the forms are the ones you built in lessons 32 and 33. What is new is the choice.

The rule

Think of it as two columns, and put every past verb into one of them.

Aorist (αόριστος) - the closed box. Use it for a single completed event (Χθες είδα τον Νίκο), for a chain of events that moves a story forward (Μπήκε, κάθισε και παρήγγειλε καφέ), and - this is the point learners miss - for a closed span: a stretch of time with a stated beginning and end, however long. Έμεινα τρία χρόνια στη Θεσσαλονίκη is aorist because the three years are counted and finished. Length is irrelevant; boundedness is everything.

Imperfect (παρατατικός) - the open line. Use it to set the scene (Έβρεχε, οι δρόμοι ήταν άδειοι), to describe a state or a person (Ήταν ψηλός και μιλούσε σιγά), for a repeated habit (Κάθε μέρα έπινε καφέ στο ίδιο μαγαζί), and for an action in progress that something else interrupts (Διάβαζα όταν χτύπησε το τηλέφωνο).

That last sentence is the model to memorise: the imperfect is the frame, the aorist is what happens inside it. English marks the same difference much more weakly, which is why the imperfect is not simply “was doing” - it also covers “used to do”, “would do” and plain “did” when the verb describes a state.

Two very common verbs escape the choice altogether. είμαι and έχω have only one past tense each, ήμουν and είχα. They are imperfect in form, but they cover the aorist reading too: Χθες ήμουν άρρωστος is both “I was ill” and “I was ill (that day, full stop)”. Do not go looking for a second form.

Finally, a few verbs change their meaning across the two columns, because the state and the moment of entering the state are different things: ήξερα is “I knew”, έμαθα is “I found out”; είχα is “I had”, απέκτησα is “I acquired”.

Forms & patterns

Nothing to learn that you have not already built. Set the two paradigms side by side and notice that only the stem differs - the imperfect is built on the continuous stem, the aorist on the simple (perfective) stem.

διαβάζω “read”

μένω “stay, live” - the pair behind the closed-span example above

είμαι and έχω - one past tense only

The same idea, two columns. With these four pairs the tense choice is also a vocabulary choice.

Note that ξέρω has no everyday aorist of its own: μαθαίνω → έμαθα supplies it.

Adverb cues. They do not decide the tense on their own, but they point very reliably.

Examples

Common mistakes

• Χθες διάβαζα το γράμμα και το πέταξα. -> Χθες διάβασα το γράμμα και το πέταξα. One finished act, followed by another: both belong in the aorist.
• Έμενα τρία χρόνια στην Κρήτη. -> Έμεινα τρία χρόνια στην Κρήτη. A counted span is a closed box, no matter how many years it holds.
• Κάθε μέρα πήγα στη δουλειά με το ποδήλατο. -> Κάθε μέρα πήγαινα στη δουλειά με το ποδήλατο. κάθε μέρα makes it a habit, so the imperfect is obligatory.
• Ήταν οκτώ, έβρεξε και ο δρόμος ήταν άδειος. -> Ήταν οκτώ, έβρεχε και ο δρόμος ήταν άδειος. Scenery is background; the aorist έβρεξε would mean one shower that started and stopped.
• Ήξερα την είδηση χθες στις δέκα. -> Έμαθα την είδηση χθες στις δέκα. With a clock time you are naming the moment the knowledge arrived, which is an event.
• Όταν ήμουν παιδί, μπόρεσα να τρέξω ώρες. -> Όταν ήμουν παιδί, μπορούσα να τρέχω ώρες. A lasting ability is a state, not a single success.
• Χθες υπήρξα στο σπίτι όλη μέρα. -> Χθες ήμουν στο σπίτι όλη μέρα. είμαι has exactly one past tense, ήμουν, and it does both jobs; the same goes for είχα.

Tips

• Before you choose, ask one question: does the sentence draw a line around the action? Line drawn, aorist. No line, imperfect.
• Picture it: the aorist is a dot or a closed box on the timeline, the imperfect is a line with no ends showing.
• In a story the two alternate by function. The imperfect paints the set (weather, place, mood, what people were like); the aorist moves the plot one step at a time.
• Watch the connector. όταν + aorist is “when, at the moment that”; ενώ and καθώς with the imperfect are “while”. Ενώ περπατούσα, συνάντησα τη Μαρία.
• Do not translate the imperfect as “was doing” and stop there. It is also “used to do”, “would do”, and plain English “did” whenever the verb describes a state: ήθελα, ήξερα, μπορούσα, νόμιζα, πίστευα.
• Learn ήξερα/έμαθα, είχα/απέκτησα, ήθελα/θέλησα, μπορούσα/μπόρεσα as four pairs of meanings, not four pairs of endings.
• Negation does not change the rule: δεν ήξερα is a state that lasted, δεν το κατάλαβα is one moment where understanding failed to arrive.