When they walking into the hall,they fall asleep.这句话从语法上看对吗

不对。
when they walked into the hall, they fell asleep.或者
when they were walking into the hall, they were asleep.

不对

有错!句子中都有主谓宾等等构成,而该句却漏了谓语,应该这样子：When they are walking into the hall,they fall asleep!