use of the demonstrative pronouns in Koine

[vir litterarum]

γρηγορεῖτε οἶν, ὅτι οὐκ οἴδατε ποίᾳ ἡμέρᾳ ὁ κύριος ὑμῶν ἔρχεται. [43] ἐκεῖνο δὲ γινώσκετε ὅτι εἰ ᾔδει ὁ οἰκοδεσπότης ποίᾳ φυλακῇ ὁ κλέπτης ἔρχεται, ἐγρηγόρησεν ἂν καὶ οὐκ ἂν εἴασεν διορυχθῆναι τὴν οὶκίαν αὐτοῦ.
Matt. 24.43-44

Why does the writer here use ἐκεῖνο and not τοῦτο?

[modus.irrealis]

It could be because of the contrast between that which they don't know and that which they are told to know (I actually read γινώσκετε as indicative at first but I don't see that making a difference). Perhaps τοῦτο can't contrast what follows to what precedes but can only be used in more neutral cases where it simply refers (either back or forward) to a statement, so it would be inappropriate here. I couldn't find any other examples in the New Testament so I'm not really sure.

[IreneY]

OK, I have to admit that I'm heavily influenced by Modern Greek in my answer but maybe that's not such a bad thing since we're talking about Koine. I think it's more or less what modus is saying; What the author wants to say is that "what you (do) know is...". Εκείνο expresses it better that τουτο.

[vir litterarum]

Look at the same verse from Luke which a commentator I found cited:

μακάριοι οἱ δοῦλοι ἐκεῖνοι, οὓς ἐλθὼν ὁ κύριος εὑρήσει γρηγοροῦντας: ἀμὴν λέγω ὑμῖν ὅτι περιζώσεται καὶ ἀνακλινεῖ αὐτοὺς καὶ παρελθὼν διακονήσει αὐτοῖς.
κἂν ἐν τῇ δευτέρᾳ κἂν ἐν τῇ τρίτῃ φυλακῇ ἔλθῃ καὶ εὕρῃ οὕτως, μακάριοί εἰσιν ἐκεῖνοι.τοῦτο δὲ γινώσκετε ὅτι εἰ ᾔδει ὁ οἰκοδεσπότης ποίᾳ ὥρᾳ ὁ κλεπτης ἔρχεται, ἐγρηγόρησεν ἂν καὶ οὐκ ἀφῆκεν διορυχθῆναι τὸν οἶκον αὐτοῦ.

You're saying the reason Luke chose τοῦτο here because there is no contrast with the preceding context of not knowing the hour in Luke's version? That makes sense; I just wasn't sure how these pronouns functioned in Koine.

In Attic, ὅτι being used to introduce a statement in apposition to a neuter pronoun would always be preceded by τοῦτο, right? In English it's translated as "know this...," not, "know that..." It just seems to me that ἐκεῖνο implies too much distance. Did the meanings of these two demonstratives change at all in Koine?

[modus.irrealis]

Here's an example from Aristotle, Nich. Ethics 1103b-1104a:

τὸ μὲν οὖν κατὰ τὸν ὀρθὸν λόγον πράττειν κοινὸν καὶ ὑποκείσθω—ῥηθήσεται δ’ ὕστερον περὶ αὐτοῦ, καὶ τί ἐστιν ὁ ὀρθὸς λόγος, καὶ πῶς ἔχει πρὸς τὰς ἄλλας ἀρετάς. ἐκεῖνο δὲ προδιομολογείσθω, ὅτι πᾶς ὁ περὶ τῶν πρακτῶν λόγος τύπῳ καὶ οὐκ ἀκριβῶς ὀφείλει λέγεσθαι, ὥσπερ καὶ κατ’ ἀρχὰς εἴπομεν ὅτι κατὰ τὴν ὕλην οἱ λόγοι ἀπαιτητέοι·

Here, there is also a contrast between what it to be discussed later and what must be assumed from the start. And about English, the Ross translation has "But this must be agreed upon beforehand, that the whole account of matters of conduct must be given in outline and not precisely..." using "this" for ἐκεῖνο. I think the details of demonstrative use vary between languages. In English "that" is probably more neutral, but I'm sure Irene would agree that in modern Greek αυτό is more neutral than εκείνο, even though it refers to what's nearer.

I also found from Xenophon, Hiero 1.19:

διὸ καὶ πάντες ἄνθρωποι ἡδέως προσδέχονται τὰς ἑορτὰς πλὴν οἱ τύραννοι· ἔκπλεῳ γὰρ αὐτοῖς ἀεὶ παρεσκευασμέναι οὐδεμίαν ἐν ταῖς ἑορταῖς ἔχουσιν αἱ τράπεζαι αὐτῶν ἐπίδοσιν· ὥστε ταύτῃ πρῶτον τῇ εὐφροσύνῃ τῆς ἐλπίδος μειονεκτοῦσι τῶν ἰδιωτῶν. ἔπειτα δ’, ἔφη, ἐκεῖνο εὖ οἶδ’ ὅτι καὶ σὺ ἔμπειρος εἶ ὅτι ὅσῳ ἂν πλείω τις παραθῆται τὰ περιττὰ τῶν ἱκανῶν, τοσούτῳ καὶ θᾶττον κόρος ἐμπίπτει τῆς ἐδωδῆς· ὥστε καὶ τῷ χρόνῳ τῆς ἡδονῆς μειονεκτεῖ ὁ παρατιθέμενος πολλὰ τῶν μετρίως διαιτωμένων.

This doesn't quite seem like contrast, but ἐκεῖνο still refers to another point in addition -- I guess again ἐκεῖνο is used to explicitly point at something that follows, instead of what precedes.

[vir litterarum]

Thanks for the examples. Isn't this usually the distinction made, in prose at least, between τοῦτο and τόδε, the former referring to preceding material and the latter to following material? What's the difference in the force of τόδε and ἐκεῖνο?

[modus.irrealis]

I've always thought the same, that it's between τοῦτο and τόδε. For ἐκεῖνο, I'm not sure. It might just be ἐκεῖνος taking over functions of ὅδε as the latter became more rare, but otherwise it would have to do something with the "distance" that the speaker felt between the thought and himself.