I must skim you must skim she must skim we must skim you must skim they must skim
V-ing form
I must be skimming you must be skimming she must be skimming we must be skimming you must be skimming they must be skimming
Perfect
I must have skimmed you must have skimmed she must have skimmed we must have skimmed you must have skimmed they must have skimmed
V-ing perfect
I must have been skimming you must have been skimming she must have been skimming we must have been skimming you must have been skimming they must have been skimming
Negation
Simple form
I mustn't skim you mustn't skim she mustn't skim we mustn't skim you mustn't skim they mustn't skim
V-ing form
I mustn't be skimming you mustn't be skimming she mustn't be skimming we mustn't be skimming you mustn't be skimming they mustn't be skimming
Perfect
I mustn't have skimmed you mustn't have skimmed she mustn't have skimmed we mustn't have skimmed you mustn't have skimmed they mustn't have skimmed
V-ing perfect
I mustn't have been skimming you mustn't have been skimming she mustn't have been skimming we mustn't have been skimming you mustn't have been skimming they mustn't have been skimming
Interrogative form
Simple form
must I skim? must you skim? must she skim? must we skim? must you skim? must they skim?
V-ing form
must I be skimming? must you be skimming? must she be skimming? must we be skimming? must you be skimming? must they be skimming?
Perfect
must I have skimmed? must you have skimmed? must she have skimmed? must we have skimmed? must you have skimmed? must they have skimmed?
V-ing perfect
must I have been skimming? must you have been skimming? must she have been skimming? must we have been skimming? must you have been skimming? must they have been skimming?
Interro-negative form
Simple form
mustn't I skim? mustn't you skim? mustn't she skim? mustn't we skim? mustn't you skim? mustn't they skim?
V-ing form
mustn't I be skimming? mustn't you be skimming? mustn't she be skimming? mustn't we be skimming? mustn't you be skimming? mustn't they be skimming?
Perfect
mustn't I have skimmed? mustn't you have skimmed? mustn't she have skimmed? mustn't we have skimmed? mustn't you have skimmed? mustn't they have skimmed?
V-ing perfect
mustn't I have been skimming? mustn't you have been skimming? mustn't she have been skimming? mustn't we have been skimming? mustn't you have been skimming? mustn't they have been skimming?