could toughen
Regular verb: toughen - toughened- toughened

Affirmation 	
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I could toughen
you could toughen
she could toughen
we could toughen
you could toughen
they could toughen 	I could be toughening
you could be toughening
she could be toughening
we could be toughening
you could be toughening
they could be toughening	I could have toughened
you could have toughened
she could have toughened
we could have toughened
you could have toughened
they could have toughened	I could have been toughening
you could have been toughening
she could have been toughening
we could have been toughening
you could have been toughening
they could have been toughening	

Negation 	
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I couldn't toughen
you couldn't toughen
she couldn't toughen
we couldn't toughen
you couldn't toughen
they couldn't toughen 	I couldn't be toughening
you couldn't be toughening
she couldn't be toughening
we couldn't be toughening
you couldn't be toughening
they couldn't be toughening	I couldn't have toughened
you couldn't have toughened
she couldn't have toughened
we couldn't have toughened
you couldn't have toughened
they couldn't have toughened	I couldn't have been toughening
you couldn't have been toughening
she couldn't have been toughening
we couldn't have been toughening
you couldn't have been toughening
they couldn't have been toughening	

Interrogative form 	
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could I toughen?
could you toughen?
could she toughen?
could we toughen?
could you toughen?
could they toughen? 	could I be toughening?
could you be toughening?
could she be toughening?
could we be toughening?
could you be toughening?
could they be toughening? 	could I have toughened?
could you have toughened?
could she have toughened?
could we have toughened?
could you have toughened?
could they have toughened? 	could I have been toughening?
could you have been toughening?
could she have been toughening?
could we have been toughening?
could you have been toughening?
could they have been toughening? 	

Interro-negative form 	
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couldn't I toughen?
couldn't you toughen?
couldn't she toughen?
couldn't we toughen?
couldn't you toughen?
couldn't they toughen? 	couldn't I be toughening?
couldn't you be toughening?
couldn't she be toughening?
couldn't we be toughening?
couldn't you be toughening?
couldn't they be toughening? 	couldn't I have toughened?
couldn't you have toughened?
couldn't she have toughened?
couldn't we have toughened?
couldn't you have toughened?
couldn't they have toughened? 	couldn't I have been toughening?
couldn't you have been toughening?
couldn't she have been toughening?
couldn't we have been toughening?
couldn't you have been toughening?
couldn't they have been toughening?