must strengthen
Regular verb: strengthen - strengthened- strengthened

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

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I must not strengthen
you must not strengthen
she must not strengthen
we must not strengthen
you must not strengthen
they must not strengthen 	I must not be strengthening
you must not be strengthening
she must not be strengthening
we must not be strengthening
you must not be strengthening
they must not be strengthening	I must not have strengthened
you must not have strengthened
she must not have strengthened
we must not have strengthened
you must not have strengthened
they must not have strengthened	I must not have been strengthening
you must not have been strengthening
she must not have been strengthening
we must not have been strengthening
you must not have been strengthening
they must not have been strengthening	

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

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