I could trek you could trek he could trek we could trek you could trek they could trek
V-ing form
I could be trekking you could be trekking he could be trekking we could be trekking you could be trekking they could be trekking
Perfect
I could have trekked you could have trekked he could have trekked we could have trekked you could have trekked they could have trekked
V-ing perfect
I could have been trekking you could have been trekking he could have been trekking we could have been trekking you could have been trekking they could have been trekking
Negation
Simple form
I could not trek you could not trek he could not trek we could not trek you could not trek they could not trek
V-ing form
I could not be trekking you could not be trekking he could not be trekking we could not be trekking you could not be trekking they could not be trekking
Perfect
I could not have trekked you could not have trekked he could not have trekked we could not have trekked you could not have trekked they could not have trekked
V-ing perfect
I could not have been trekking you could not have been trekking he could not have been trekking we could not have been trekking you could not have been trekking they could not have been trekking
Interrogative form
Simple form
could I trek? could you trek? could he trek? could we trek? could you trek? could they trek?
V-ing form
could I be trekking? could you be trekking? could he be trekking? could we be trekking? could you be trekking? could they be trekking?
Perfect
could I have trekked? could you have trekked? could he have trekked? could we have trekked? could you have trekked? could they have trekked?
V-ing perfect
could I have been trekking? could you have been trekking? could he have been trekking? could we have been trekking? could you have been trekking? could they have been trekking?
Interro-negative form
Simple form
could I not trek? could you not trek? could he not trek? could we not trek? could you not trek? could they not trek?
V-ing form
could I not be trekking? could you not be trekking? could he not be trekking? could we not be trekking? could you not be trekking? could they not be trekking?
Perfect
could I not have trekked? could you not have trekked? could he not have trekked? could we not have trekked? could you not have trekked? could they not have trekked?
V-ing perfect
could I not have been trekking? could you not have been trekking? could he not have been trekking? could we not have been trekking? could you not have been trekking? could they not have been trekking?