must strengthen
Regular verb: strengthen - strengthened- strengthened

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I must strengthen
you must strengthen
he must strengthen
we must strengthen
you must strengthen
they must strengthen 	I must be strengthening
you must be strengthening
he must be strengthening
we must be strengthening
you must be strengthening
they must be strengthening	I must have strengthened
you must have strengthened
he 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
he 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 mustn't strengthen
you mustn't strengthen
he mustn't strengthen
we mustn't strengthen
you mustn't strengthen
they mustn't strengthen 	I mustn't be strengthening
you mustn't be strengthening
he mustn't be strengthening
we mustn't be strengthening
you mustn't be strengthening
they mustn't be strengthening	I mustn't have strengthened
you mustn't have strengthened
he mustn't have strengthened
we mustn't have strengthened
you mustn't have strengthened
they mustn't have strengthened	I mustn't have been strengthening
you mustn't have been strengthening
he mustn't have been strengthening
we mustn't have been strengthening
you mustn't have been strengthening
they mustn't have been strengthening	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
must I strengthen?
must you strengthen?
must he strengthen?
must we strengthen?
must you strengthen?
must they strengthen? 	must I be strengthening?
must you be strengthening?
must he be strengthening?
must we be strengthening?
must you be strengthening?
must they be strengthening? 	must I have strengthened?
must you have strengthened?
must he 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 he 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 	
mustn't I strengthen?
mustn't you strengthen?
mustn't he strengthen?
mustn't we strengthen?
mustn't you strengthen?
mustn't they strengthen? 	mustn't I be strengthening?
mustn't you be strengthening?
mustn't he be strengthening?
mustn't we be strengthening?
mustn't you be strengthening?
mustn't they be strengthening? 	mustn't I have strengthened?
mustn't you have strengthened?
mustn't he have strengthened?
mustn't we have strengthened?
mustn't you have strengthened?
mustn't they have strengthened? 	mustn't I have been strengthening?
mustn't you have been strengthening?
mustn't he have been strengthening?
mustn't we have been strengthening?
mustn't you have been strengthening?
mustn't they have been strengthening?