Matt. 5:25

[vir litterarum]

[modus.irrealis]

I'd say that εως here is being used as a preposition and οτου refers back to some implied noun meaning "time" or something, so literally εως οτου is something like "[in the time] during which" but it's pretty much equivalent as a whole to "while."

[Bert]

I agree. The word is translated but together with ἕως. Until whichever (time) -> as long as, while.

[vir litterarum]

So it would be a genitive of time at which the clause occurs?

[Bert]

So it would be a genitive of time at which the clause occurs?

No, it is genitive because here ἕως is a preposition that takes its object in the genitive.

[vir litterarum]

Are you sure "ews" can be used as a preposition? I looked my middle LSJ and saw no mention of it.

[modus.irrealis]

[Bert]