"Please don't ask difficult questions."
難しい質問をしないで下さい。(Continuing a previous commenter's habit of adding the kanji version of the sentence for review or reference for others since it's more common than writing words out in hiragana.)
I had the same confusion before. My Japanese teacher said that 聞く is used without the stated 質問. When you want to use just the verb, like ask about money, you could say お金について聞く。But when you want to state the word ''question,'' use する with しつもん。 So ''ask a QUESTION'' would be しつもんをする but simply ''ask'' would be きく。
Ohhh thanks for clarifying. I was a confused not seeing the word for "ask," so I thought I'd used the word for "do" instead and got it right. I thought "do a question?"
In spoken Japanese it is often omitted, but for writing it needs to be included.
Is it also possible to say...
I thought that 聞く was "to ask" when paired with the に particle.
聞く does mean "to ask", but if you're using it with に you need the person you're asking in front. 先生に聞く = ask the teacher
質問 is a noun, but you can think of 質問(を)する as either a verb in itself, or of する as the verb that goes with 質問. You wouldn't generally say 質問を聞く in Japanese.