I could grade you could grade he could grade we could grade you could grade they could grade
V-ing form
I could be grading you could be grading he could be grading we could be grading you could be grading they could be grading
Perfect
I could have graded you could have graded he could have graded we could have graded you could have graded they could have graded
V-ing perfect
I could have been grading you could have been grading he could have been grading we could have been grading you could have been grading they could have been grading
Negation
Simple form
I could not grade you could not grade he could not grade we could not grade you could not grade they could not grade
V-ing form
I could not be grading you could not be grading he could not be grading we could not be grading you could not be grading they could not be grading
Perfect
I could not have graded you could not have graded he could not have graded we could not have graded you could not have graded they could not have graded
V-ing perfect
I could not have been grading you could not have been grading he could not have been grading we could not have been grading you could not have been grading they could not have been grading
Interrogative form
Simple form
could I grade? could you grade? could he grade? could we grade? could you grade? could they grade?
V-ing form
could I be grading? could you be grading? could he be grading? could we be grading? could you be grading? could they be grading?
Perfect
could I have graded? could you have graded? could he have graded? could we have graded? could you have graded? could they have graded?
V-ing perfect
could I have been grading? could you have been grading? could he have been grading? could we have been grading? could you have been grading? could they have been grading?
Interro-negative form
Simple form
could I not grade? could you not grade? could he not grade? could we not grade? could you not grade? could they not grade?
V-ing form
could I not be grading? could you not be grading? could he not be grading? could we not be grading? could you not be grading? could they not be grading?
Perfect
could I not have graded? could you not have graded? could he not have graded? could we not have graded? could you not have graded? could they not have graded?
V-ing perfect
could I not have been grading? could you not have been grading? could he not have been grading? could we not have been grading? could you not have been grading? could they not have been grading?