1) May I ask: [...your question...] ; May I know [...your question...]?

2) May I ask where are you going? May I know when you are going to marry me?

3) Please advise: Can we deliver you the goods by next Friday?

4) Please let me know: When are you going to pay us the money?

5) (If the teacher wants students to "ask questions"): Please ask now [Please (now) ask]. Please ask your question now.