could stem
Regular verb: stem - stemmed- stemmed

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could stem
you could stem
she could stem
we could stem
you could stem
they could stem 	I could be stemming
you could be stemming
she could be stemming
we could be stemming
you could be stemming
they could be stemming	I could have stemmed
you could have stemmed
she could have stemmed
we could have stemmed
you could have stemmed
they could have stemmed	I could have been stemming
you could have been stemming
she could have been stemming
we could have been stemming
you could have been stemming
they could have been stemming	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could not stem
you could not stem
she could not stem
we could not stem
you could not stem
they could not stem 	I could not be stemming
you could not be stemming
she could not be stemming
we could not be stemming
you could not be stemming
they could not be stemming	I could not have stemmed
you could not have stemmed
she could not have stemmed
we could not have stemmed
you could not have stemmed
they could not have stemmed	I could not have been stemming
you could not have been stemming
she could not have been stemming
we could not have been stemming
you could not have been stemming
they could not have been stemming	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I stem?
could you stem?
could she stem?
could we stem?
could you stem?
could they stem? 	could I be stemming?
could you be stemming?
could she be stemming?
could we be stemming?
could you be stemming?
could they be stemming? 	could I have stemmed?
could you have stemmed?
could she have stemmed?
could we have stemmed?
could you have stemmed?
could they have stemmed? 	could I have been stemming?
could you have been stemming?
could she have been stemming?
could we have been stemming?
could you have been stemming?
could they have been stemming? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I not stem?
could you not stem?
could she not stem?
could we not stem?
could you not stem?
could they not stem? 	could I not be stemming?
could you not be stemming?
could she not be stemming?
could we not be stemming?
could you not be stemming?
could they not be stemming? 	could I not have stemmed?
could you not have stemmed?
could she not have stemmed?
could we not have stemmed?
could you not have stemmed?
could they not have stemmed? 	could I not have been stemming?
could you not have been stemming?
could she not have been stemming?
could we not have been stemming?
could you not have been stemming?
could they not have been stemming?