There is no knowing what may happen.

没人知道知道会发生什么事。

There is no knowing what he is doing.

无法知道他在做什么。

There is no knowing when he will come.

的翻译是：没有知道他何时将来。

There is no knowing when he can finish the book.

无法知道他何时才能写完这本书。

There is no knowing what may happen tomorrow.

谁也不知道明天将发生什么。