I must lack you must lack she must lack we must lack you must lack they must lack
V-ing form
I must be lacking you must be lacking she must be lacking we must be lacking you must be lacking they must be lacking
Perfect
I must have lacked you must have lacked she must have lacked we must have lacked you must have lacked they must have lacked
V-ing perfect
I must have been lacking you must have been lacking she must have been lacking we must have been lacking you must have been lacking they must have been lacking
Negation
Simple form
I must not lack you must not lack she must not lack we must not lack you must not lack they must not lack
V-ing form
I must not be lacking you must not be lacking she must not be lacking we must not be lacking you must not be lacking they must not be lacking
Perfect
I must not have lacked you must not have lacked she must not have lacked we must not have lacked you must not have lacked they must not have lacked
V-ing perfect
I must not have been lacking you must not have been lacking she must not have been lacking we must not have been lacking you must not have been lacking they must not have been lacking
Interrogative form
Simple form
must I lack? must you lack? must she lack? must we lack? must you lack? must they lack?
V-ing form
must I be lacking? must you be lacking? must she be lacking? must we be lacking? must you be lacking? must they be lacking?
Perfect
must I have lacked? must you have lacked? must she have lacked? must we have lacked? must you have lacked? must they have lacked?
V-ing perfect
must I have been lacking? must you have been lacking? must she have been lacking? must we have been lacking? must you have been lacking? must they have been lacking?
Interro-negative form
Simple form
must I not lack? must you not lack? must she not lack? must we not lack? must you not lack? must they not lack?
V-ing form
must I not be lacking? must you not be lacking? must she not be lacking? must we not be lacking? must you not be lacking? must they not be lacking?
Perfect
must I not have lacked? must you not have lacked? must she not have lacked? must we not have lacked? must you not have lacked? must they not have lacked?
V-ing perfect
must I not have been lacking? must you not have been lacking? must she not have been lacking? must we not have been lacking? must you not have been lacking? must they not have been lacking?