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 couldn't trek you couldn't trek he couldn't trek we couldn't trek you couldn't trek they couldn't trek
V-ing form
I couldn't be trekking you couldn't be trekking he couldn't be trekking we couldn't be trekking you couldn't be trekking they couldn't be trekking
Perfect
I couldn't have trekked you couldn't have trekked he couldn't have trekked we couldn't have trekked you couldn't have trekked they couldn't have trekked
V-ing perfect
I couldn't have been trekking you couldn't have been trekking he couldn't have been trekking we couldn't have been trekking you couldn't have been trekking they couldn't 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
couldn't I trek? couldn't you trek? couldn't he trek? couldn't we trek? couldn't you trek? couldn't they trek?
V-ing form
couldn't I be trekking? couldn't you be trekking? couldn't he be trekking? couldn't we be trekking? couldn't you be trekking? couldn't they be trekking?
Perfect
couldn't I have trekked? couldn't you have trekked? couldn't he have trekked? couldn't we have trekked? couldn't you have trekked? couldn't they have trekked?
V-ing perfect
couldn't I have been trekking? couldn't you have been trekking? couldn't he have been trekking? couldn't we have been trekking? couldn't you have been trekking? couldn't they have been trekking?