You mustn't pick the strawberries in the garden.
Where must you pick the strawberries?
Mark needn't take the umbrella with him.
Who must take the umbrella with him?
You mustn't go out at eight o'clock.
What time must you go out?
You mustn't open the door at night.
What must you do at night?
must是情态动词,只有一种形式,表示必须,它的过去式和现在式（原形）同型,没有分词形式.希望我能帮助你解疑释惑.