The key must lie in sleep's psychological component.请问句中的must是什么词性?是助动词吗?能举几个must做助动词和情态动词的例句吗？

The key must lie in sleep's psychological component.请问句中的must是什么词性?是助动词吗?能举几个must做助动词和情态动词的例句吗？ lie in the north lie in the fact lie in the fact lie in the fact 适当形式填空What he said is not ture.He is __(lie).There must be a key in your pocket,___ ___?(完成反义疑问句) The key lies in the fact that the untis of meaning这一句里的lie in 为什么不能不用hide inThe key lies in the fact that the untis of meaning这一句里的lie in 为什么不能不用hides in?这是去年大学英语四级完型填空,答 Next key in the numbers Now their talks have resched a key stagenow their talks have reached a key stage where one side must give in to the other.为什么用where? lie in the bed还是lie on the bed? the key find I must .连词成句 I want to know why you ( )after him on the street yesterday Because I must give the key to him .He couldn't get in to the room --Where she have put the key to the door?--- It maybe in the draw.A.can B.may C.must D.should lie in the north的意思 lie in the sun怎么解释 lie in the hands of children是什么意思 _____the dog_____(lie)in the floor now? He has a headache,so he must （）for a rest.A.lie in B.lie up C.lie under D.lie down 原因