could strengthen
Regular verb: strengthen - strengthened- strengthened

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could strengthen
you could strengthen
she could strengthen
we could strengthen
you could strengthen
they could strengthen 	I could be strengthening
you could be strengthening
she could be strengthening
we could be strengthening
you could be strengthening
they could be strengthening	I could have strengthened
you could have strengthened
she could have strengthened
we could have strengthened
you could have strengthened
they could have strengthened	I could have been strengthening
you could have been strengthening
she could have been strengthening
we could have been strengthening
you could have been strengthening
they could have been strengthening	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I couldn't strengthen
you couldn't strengthen
she couldn't strengthen
we couldn't strengthen
you couldn't strengthen
they couldn't strengthen 	I couldn't be strengthening
you couldn't be strengthening
she couldn't be strengthening
we couldn't be strengthening
you couldn't be strengthening
they couldn't be strengthening	I couldn't have strengthened
you couldn't have strengthened
she couldn't have strengthened
we couldn't have strengthened
you couldn't have strengthened
they couldn't have strengthened	I couldn't have been strengthening
you couldn't have been strengthening
she couldn't have been strengthening
we couldn't have been strengthening
you couldn't have been strengthening
they couldn't have been strengthening	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I strengthen?
could you strengthen?
could she strengthen?
could we strengthen?
could you strengthen?
could they strengthen? 	could I be strengthening?
could you be strengthening?
could she be strengthening?
could we be strengthening?
could you be strengthening?
could they be strengthening? 	could I have strengthened?
could you have strengthened?
could she have strengthened?
could we have strengthened?
could you have strengthened?
could they have strengthened? 	could I have been strengthening?
could you have been strengthening?
could she have been strengthening?
could we have been strengthening?
could you have been strengthening?
could they have been strengthening? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
couldn't I strengthen?
couldn't you strengthen?
couldn't she strengthen?
couldn't we strengthen?
couldn't you strengthen?
couldn't they strengthen? 	couldn't I be strengthening?
couldn't you be strengthening?
couldn't she be strengthening?
couldn't we be strengthening?
couldn't you be strengthening?
couldn't they be strengthening? 	couldn't I have strengthened?
couldn't you have strengthened?
couldn't she have strengthened?
couldn't we have strengthened?
couldn't you have strengthened?
couldn't they have strengthened? 	couldn't I have been strengthening?
couldn't you have been strengthening?
couldn't she have been strengthening?
couldn't we have been strengthening?
couldn't you have been strengthening?
couldn't they have been strengthening?