They went on from there and passed through Galilee. He did not want anyone to know it; for he was teaching his disciples, saying to them, ‘The Son of Man is to be betrayed into human hands, and they will kill him, and three days after being killed, he will rise again.’ But they did not understand what he was saying and were afraid to ask him. (Mark 9: 30-32)

Certainly seems clear enough. Pretty specific too: after three days. Betrayed, then killed, then rise. What is not to understand?

The mystery is not in what Jesus says, but in how the disciples respond. Matthew and Luke are clearly paraphrasing Mark and do not add to our understanding.

Mark and Luke explain they were afraid. The Greek suggests being startled or amazed by something unexpected.

The Greek also implies a deferential - reverential - attitude. They did not ask out of concern for being rude or being exposed as stupid.

Jesus had recently been direct - even harsh - in critique. Why risk asking? They did not trust Jesus. Which was exactly the critique Jesus had made.