μπόρεσα=simple past, μπορούσα=past continuous. Only Θα + past continuous confers the subjective (would + verb)
The English sentence could be a simple past, though, since "could" has two meanings. "Yesterday I could eat more, but today I have a stomach ache."
The English sentence could, but the Greek translation couldn't. . To put this simpler, μπόρεσα : was able to, μπορούσα : could. If your sentence can't use "was able to" instead of could, then you need continuous. Hope that helps!
I had it as an English to Greek. Fortunately, I didn't know the past tense of the verb, so got it right.
why doesn't μπορεσα work here?
Oh, additionally, there is no verb form with θα and simple past (αόριστος), only continuous (παρατατικός).
What would θα μπορώ να τρώω περισσότερο mean
I will be able to eat more. :)