We can never know what would have happened. (talking about something in past) -> Looks fine! We can never know what will happen. (talking about something in future) -> What is going to happen is more frequently used but this is fine :) We can never know what's going to happen. (talking about something in future) -> Fine too :) ******************************************************************************** NOTE : In my opinion, "we will never know" is more often used. Also, the first sentence is a dialogue in "The Chronicles of Narnia: Prince Caspian" spoken by Aslan :D ******************************************************************************** for Past you can use : "We never know what could / would have happened" For future : "We can never predict what will happen" ********************************************************************************

I would like to add "beforehand" into the futuretime sentences...I am not sure though if it is absolutely necessary... Nevertheless, I might change the last sentence in to: "We can never know what is about to happen." = to be going to happen very soon