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