I could sit you could sit he could sit we could sit you could sit they could sit
V-ing form
I could be sitting you could be sitting he could be sitting we could be sitting you could be sitting they could be sitting
Perfect
I could have sat you could have sat he could have sat we could have sat you could have sat they could have sat
V-ing perfect
I could have been sitting you could have been sitting he could have been sitting we could have been sitting you could have been sitting they could have been sitting
Negation
Simple form
I couldn't sit you couldn't sit he couldn't sit we couldn't sit you couldn't sit they couldn't sit
V-ing form
I couldn't be sitting you couldn't be sitting he couldn't be sitting we couldn't be sitting you couldn't be sitting they couldn't be sitting
Perfect
I couldn't have sat you couldn't have sat he couldn't have sat we couldn't have sat you couldn't have sat they couldn't have sat
V-ing perfect
I couldn't have been sitting you couldn't have been sitting he couldn't have been sitting we couldn't have been sitting you couldn't have been sitting they couldn't have been sitting
Interrogative form
Simple form
could I sit? could you sit? could he sit? could we sit? could you sit? could they sit?
V-ing form
could I be sitting? could you be sitting? could he be sitting? could we be sitting? could you be sitting? could they be sitting?
Perfect
could I have sat? could you have sat? could he have sat? could we have sat? could you have sat? could they have sat?
V-ing perfect
could I have been sitting? could you have been sitting? could he have been sitting? could we have been sitting? could you have been sitting? could they have been sitting?
Interro-negative form
Simple form
couldn't I sit? couldn't you sit? couldn't he sit? couldn't we sit? couldn't you sit? couldn't they sit?
V-ing form
couldn't I be sitting? couldn't you be sitting? couldn't he be sitting? couldn't we be sitting? couldn't you be sitting? couldn't they be sitting?
Perfect
couldn't I have sat? couldn't you have sat? couldn't he have sat? couldn't we have sat? couldn't you have sat? couldn't they have sat?
V-ing perfect
couldn't I have been sitting? couldn't you have been sitting? couldn't he have been sitting? couldn't we have been sitting? couldn't you have been sitting? couldn't they have been sitting?