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