"Buddha wrote:Udayin, if someone should recollect his manifold past lives, that is, one birth, two births....thus, with their aspects and particulars, should he recollect his manifold past lives, then either he might ask me a question about the past or I might ask him a question about the past, and he might satisfy my mind with his answer to my question or I might satisfy his mind with my answer to his question. If someone with the divine eye, which is purified and surpasses the human, would see beings passing away and reappearing, inferior and superior, fair and ugly, fortunate and unfortunate...and understand how beings pass on according to their actions, then either he might ask me a question about the future or I might ask him a question about the future, and he might satisfy my mind with his answer to my question or I might satisfy his mind with my answer to his question. But let be the past, Udayin, let be the future. I shall teach you the Dhamma: When this exists, that comes to be; with the arising of this, that arises. When this does not exist, that does not come to be; with the cessation of this,that ceases.
From what I see here, it appears the Buddha is saying, "Asking me a question about past lives when you yourself cannot see past lives will likely result in you being unsatisfied with any answer I give you. Likewise, asking me a question about the future when you yourself cannot see kamma will likely result in similar dissatisfaction. So don't ask me such questions." But then we have MN 57 in which two ascetics, Punya and Seniya, ask the Buddha about the future and the Buddha answers them. In fact there are many such conversations in the sutta with ascetics, with monks, and with laypeople too. So why in this sutta does the Buddha say "let be the past, let be the future"? I can think of three reasons for the discrepancy between these two suttas.
a] Perhaps Punya and Seniya could see kamma, therefore the Buddha would talk to them about the future? I would guess not. Specifically these ascetics asked what would be the future course for one who does a certain practice. If they could have seen kamma for themselves, I would think they could have answered their own question. Perhaps they had some ability to see kamma but not enough to answer their own question?
b] The specifics of the questions in these two suttas were different. One was worthy of answering and the other was not. But looking at the rest of MN 79 we never see what Udayin's question was. In fact, the sutta doesn't seem to actually be about Udayin's question but rather about the fact that Udayin asked a question of another teacher and was unsatisfied with the answer.
c] There was a difference between the two people. For one the question was worth answering and for the other it was not worth answering. The suttas don't really tell us anything about the people asking the questions. Maybe the commentaries do? One difference the suttas do make clear is Punya and Seniya were satisfied by the Buddha's answer and Udayin was unsatisfied with him answer. Perhaps the difference is Punya and Seniya were believers in rebirth and Udayin was a skeptic? Perhaps the Buddha is saying "If you are skeptical of rebirth then don't bother asking questions about it"?
I don't know. It's an interesting sutta. The other teacher Udayin saw was Nigantha Nataputta, someone the Buddha usually doesn't hesitate to show up. In this case though it seems like the Buddha was defending him. "You were dissatisfied with Niatha Nataputta's answer not because any lack of his but through your own lack."