Possibility and Permission: μπορεί / μπορώ

The present tense of μπορώ is settled elsewhere: the personal μπορώ να ("I'm able to") against the frozen impersonal μπορεί να ("it might") is the subject of impersonal πρέπει & μπορεί να, and the full paradigm sits on the μπορώ reference page. What this page owns is everything to the left and right of the present: the past μπορούσα, its aorist rival μπόρεσα, the conditional θα μπορούσα, and the stacked θα μπορούσα να είχα…. Between them they carry almost every English sentence with could.

μπορούσα — the past "was able to"

μπορούσα is the imperfect of μπορώ, and it describes an ability or an open possibility that held over a stretch of past time. Think of it as a window that was open: while it was open, the thing was doable.

μπορούσα against μπόρεσα: "could" vs "managed to"

English pours two different things into could, and Greek keeps them apart — this is the single most useful distinction on the page.

• μπορούσα (imperfect) — the ability or possibility existed. Whether it was ever cashed in is not part of the claim.
• μπόρεσα (aorist) — on that one occasion, the thing came off (or, negated, did not come off despite the attempt).

Line the two negatives up and the difference is sharp. Δεν μπορούσα να κοιμηθώ describes a state that lasted all night; Δεν μπόρεσα να έρθω reports one failed occasion. Pick the imperfect for a background condition, the aorist for a single outcome. (The imperfect has a second, entirely non-past job as a politeness softener — Μπορούσατε να μου δείξετε…; — which belongs to the imperfect for politeness.)

θα μπορούσα — the conditional "could"

Put θα in front of μπορούσα and you get the conditional: θα μπορούσα, θα μπορούσες, θα μπορούσε… — "could / would be able to". The θα-machinery itself is on the present conditional; what matters here is how much everyday work this one form does.

Hypothetical ability, with the condition stated or merely implied:

A suggestion — this is the everyday "we could…", and it is softer than a proposal with να or ας:

A reproach, which in Greek as in English is a suggestion aimed backwards:

A hedged claim (formal, academic) — θα μπορούσε κανείς να πει… is the standard way of advancing an argument without committing to it:

And a fixed consolation, worth learning whole:

For requests — Θα μπορούσατε να…; — θα μπορούσα is the top rung of the politeness ladder; that ladder is laid out on softening commands and the conditional for politeness, so it is not repeated here.

θα μπορώ is not θα μπορούσα

One trap that costs learners real misunderstandings: θα μπορώ / θα μπορέσω is a future ("I will be able to"), a promise about a time that has not arrived. θα μπορούσα is a conditional ("I could"), an offer or hypothesis about now. English "I'll be able to" and "I could" are equally far apart — it is only the shared θα that makes them look like relatives.

Δεν θα μπορούσα — more than a plain negative

Negated, the conditional is usually doing more than reporting an inability. Δεν θα μπορούσα is an emphatic statement of impossibility, and in social use it is the standard way of declining to impose on someone — the Greek equivalent of "oh, I couldn't possibly".

With a third-person subject it becomes a verdict on how things stood, close to English "it couldn't have been otherwise":

θα μπορούσα να είχα… — "I could have"

For a possibility that is over and did not happen, Greek layers the two pieces you already have: θα μπορούσα supplies "would have been able", and να είχα + the -ει form supplies "already done". The pluperfect half of that — and the θα είχα counterfactual it is built on — is the business of the pluperfect.

Be aware that Greek has three shapes circulating for this, and they differ in explicitness rather than in grammaticality. Θα μπορούσα να πάω leans on a past context to be understood as "I could have gone"; Μπορούσα να είχα πάει drops the θα and is common in speech; Θα μπορούσα να είχα πάει spells everything out and is what you want in writing. All three are current — reach for the fullest one when the sentence has to stand on its own.

Common Mistakes

1. Using the future where you mean the conditional. Both are grammatical; they commit you to very different things.

2. Putting a past tense straight after θα μπορούσα. The conditional takes the dependent (να πάω) or the past subjunctive (να είχα πάει). A bare past tense after να belongs to a different frame — the impersonal guess μπορεί να πήγα ("I may have gone") — and it does not combine with θα μπορούσα.

3. Telling a past story with the wrong "couldn't". A failed single attempt is the aorist; a condition that held for a while is the imperfect.

4. Offering help in the present when the offer is already too late. English keeps could and could have one word apart, and learners let the first cover both.

Key Takeaways

• μπορούσα = an ability or possibility that held over past time; μπόρεσα = "managed to" on one occasion. English could covers both, so the choice is yours to make.
• Δεν μπορούσα describes a lasting condition; Δεν μπόρεσα reports one failed occasion.
• θα μπορούσα is the everyday "could": hypothetical ability, suggestions (Θα μπορούσαμε να…), polite requests (Θα μπορούσατε να…;), reproach (Θα μπορούσες τουλάχιστον να…), hedged claims (academic) and the fixed θα μπορούσε να είναι χειρότερα.
• Negated, δεν θα μπορούσα is emphatic impossibility and the polite way to refuse an imposition ("I couldn't possibly").
• Do not confuse θα μπορώ / θα μπορέσω ("I'll be able to", a future promise) with θα μπορούσα ("I could", a conditional offer).
• θα μπορούσα να είχα + -ει = "I could have…", the chance that was not taken; the lighter μπορούσα να είχα… and the context-dependent θα μπορούσα να… say the same thing with less machinery.