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