I could mark you could mark he could mark we could mark you could mark they could mark
V-ing form
I could be marking you could be marking he could be marking we could be marking you could be marking they could be marking
Perfect
I could have marked you could have marked he could have marked we could have marked you could have marked they could have marked
V-ing perfect
I could have been marking you could have been marking he could have been marking we could have been marking you could have been marking they could have been marking
Negation
Simple form
I could not mark you could not mark he could not mark we could not mark you could not mark they could not mark
V-ing form
I could not be marking you could not be marking he could not be marking we could not be marking you could not be marking they could not be marking
Perfect
I could not have marked you could not have marked he could not have marked we could not have marked you could not have marked they could not have marked
V-ing perfect
I could not have been marking you could not have been marking he could not have been marking we could not have been marking you could not have been marking they could not have been marking
Interrogative form
Simple form
could I mark? could you mark? could he mark? could we mark? could you mark? could they mark?
V-ing form
could I be marking? could you be marking? could he be marking? could we be marking? could you be marking? could they be marking?
Perfect
could I have marked? could you have marked? could he have marked? could we have marked? could you have marked? could they have marked?
V-ing perfect
could I have been marking? could you have been marking? could he have been marking? could we have been marking? could you have been marking? could they have been marking?
Interro-negative form
Simple form
could I not mark? could you not mark? could he not mark? could we not mark? could you not mark? could they not mark?
V-ing form
could I not be marking? could you not be marking? could he not be marking? could we not be marking? could you not be marking? could they not be marking?
Perfect
could I not have marked? could you not have marked? could he not have marked? could we not have marked? could you not have marked? could they not have marked?
V-ing perfect
could I not have been marking? could you not have been marking? could he not have been marking? could we not have been marking? could you not have been marking? could they not have been marking?