there mustbe someone doing sth是什么结构

答：There must be someone doing sth是属于there be 结构的情态动词形式的。它表示一种肯定的推测，意思是：此时/现在一定是/准是/肯定是有某人正在做某事的。例如：

Look! The light in the office is still on. There must be someone working now. 瞧！办公室的灯仍然亮着。现在肯定是还有人在那里加班做工的。