The preterist interpretation of the Olivet discourse rests heavily on Matthew 24:34. Mathison says,
The key to understanding the entire discourse is found in verse 34, in which Jesus tells His disciples, “Truly I say to you, this generation will not pass away until all these things take place.” Jesus declares that his prophecy will be fulfilled before the generation to whim He is speaking passes away. In other words, the events of which he speaks in this passage will be fulfilled by A.D. 70, one generation from the date He made the pronouncement.”
Keith A. Mathison, Postmillennialism: An Eschatology of Hope, 111.
There are a number of hard passages for the preterist within the discourse (see Mathison 112-15 for his explanation of them), but Matthew 24:34 is the most difficult for the non-preterist. Bavinck’s explanation of Matthew 24:34 makes good sense:
The words “this generation” (ἡ γενεα αὑτη, hē genea hautē) cannot be understood to mean the Jewish people, but undoubtedly refer to the generation then living. On the other hand, it is clear that the words ‘all these things’ (παντα τυατα, panta tauta) do not include the parousia itself but only refer to the signs that precede and announce it. For after predicting the destruction of Jerusalem and the signs and his return and even the gathering of his elect by the angles, and therefore actually ending his eschatological discourse, Jesus proceeds in verse 32 to offer a practical application. Here he states that just as in the case of the fig tree the sprouting of the leaves announces the summer, so ‘all these things’ are signs that the end is near or that the Messiah is at the door. Here the expression panta tauta clearly refers to the signs of the coming parousia, not to the parousia itself, for else it would make no sense to say that when ‘these things’ occur, the end is ‘near.’ In verse 34 the words ‘all these things’ (panta tauta) have the same meaning. Jesus therefore does not say that his parousia will still occur within the time of the generation then living. What he says is that the signs and portents of it, as they would be visible in the destruction of Jerusalem and concomitant events, would begin to occur in the time of the generation then living.
Bavinck, Reformed Dogmatics, 4:687.