I must occur you must occur he must occur we must occur you must occur they must occur
V-ing form
I must be occurring you must be occurring he must be occurring we must be occurring you must be occurring they must be occurring
Perfect
I must have occurred you must have occurred he must have occurred we must have occurred you must have occurred they must have occurred
V-ing perfect
I must have been occurring you must have been occurring he must have been occurring we must have been occurring you must have been occurring they must have been occurring
Negation
Simple form
I mustn't occur you mustn't occur he mustn't occur we mustn't occur you mustn't occur they mustn't occur
V-ing form
I mustn't be occurring you mustn't be occurring he mustn't be occurring we mustn't be occurring you mustn't be occurring they mustn't be occurring
Perfect
I mustn't have occurred you mustn't have occurred he mustn't have occurred we mustn't have occurred you mustn't have occurred they mustn't have occurred
V-ing perfect
I mustn't have been occurring you mustn't have been occurring he mustn't have been occurring we mustn't have been occurring you mustn't have been occurring they mustn't have been occurring
Interrogative form
Simple form
must I occur? must you occur? must he occur? must we occur? must you occur? must they occur?
V-ing form
must I be occurring? must you be occurring? must he be occurring? must we be occurring? must you be occurring? must they be occurring?
Perfect
must I have occurred? must you have occurred? must he have occurred? must we have occurred? must you have occurred? must they have occurred?
V-ing perfect
must I have been occurring? must you have been occurring? must he have been occurring? must we have been occurring? must you have been occurring? must they have been occurring?
Interro-negative form
Simple form
mustn't I occur? mustn't you occur? mustn't he occur? mustn't we occur? mustn't you occur? mustn't they occur?
V-ing form
mustn't I be occurring? mustn't you be occurring? mustn't he be occurring? mustn't we be occurring? mustn't you be occurring? mustn't they be occurring?
Perfect
mustn't I have occurred? mustn't you have occurred? mustn't he have occurred? mustn't we have occurred? mustn't you have occurred? mustn't they have occurred?
V-ing perfect
mustn't I have been occurring? mustn't you have been occurring? mustn't he have been occurring? mustn't we have been occurring? mustn't you have been occurring? mustn't they have been occurring?