I must err you must err she must err we must err you must err they must err
V-ing form
I must be erring you must be erring she must be erring we must be erring you must be erring they must be erring
Perfect
I must have erred you must have erred she must have erred we must have erred you must have erred they must have erred
V-ing perfect
I must have been erring you must have been erring she must have been erring we must have been erring you must have been erring they must have been erring
Negation
Simple form
I must not err you must not err she must not err we must not err you must not err they must not err
V-ing form
I must not be erring you must not be erring she must not be erring we must not be erring you must not be erring they must not be erring
Perfect
I must not have erred you must not have erred she must not have erred we must not have erred you must not have erred they must not have erred
V-ing perfect
I must not have been erring you must not have been erring she must not have been erring we must not have been erring you must not have been erring they must not have been erring
Interrogative form
Simple form
must I err? must you err? must she err? must we err? must you err? must they err?
V-ing form
must I be erring? must you be erring? must she be erring? must we be erring? must you be erring? must they be erring?
Perfect
must I have erred? must you have erred? must she have erred? must we have erred? must you have erred? must they have erred?
V-ing perfect
must I have been erring? must you have been erring? must she have been erring? must we have been erring? must you have been erring? must they have been erring?
Interro-negative form
Simple form
must I not err? must you not err? must she not err? must we not err? must you not err? must they not err?
V-ing form
must I not be erring? must you not be erring? must she not be erring? must we not be erring? must you not be erring? must they not be erring?
Perfect
must I not have erred? must you not have erred? must she not have erred? must we not have erred? must you not have erred? must they not have erred?
V-ing perfect
must I not have been erring? must you not have been erring? must she not have been erring? must we not have been erring? must you not have been erring? must they not have been erring?