must underpin
Regular verb: underpin - underpinned- underpinned

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I must underpin
you must underpin
he must underpin
we must underpin
you must underpin
they must underpin 	I must be underpinning
you must be underpinning
he must be underpinning
we must be underpinning
you must be underpinning
they must be underpinning	I must have underpinned
you must have underpinned
he must have underpinned
we must have underpinned
you must have underpinned
they must have underpinned	I must have been underpinning
you must have been underpinning
he must have been underpinning
we must have been underpinning
you must have been underpinning
they must have been underpinning	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I mustn't underpin
you mustn't underpin
he mustn't underpin
we mustn't underpin
you mustn't underpin
they mustn't underpin 	I mustn't be underpinning
you mustn't be underpinning
he mustn't be underpinning
we mustn't be underpinning
you mustn't be underpinning
they mustn't be underpinning	I mustn't have underpinned
you mustn't have underpinned
he mustn't have underpinned
we mustn't have underpinned
you mustn't have underpinned
they mustn't have underpinned	I mustn't have been underpinning
you mustn't have been underpinning
he mustn't have been underpinning
we mustn't have been underpinning
you mustn't have been underpinning
they mustn't have been underpinning	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
must I underpin?
must you underpin?
must he underpin?
must we underpin?
must you underpin?
must they underpin? 	must I be underpinning?
must you be underpinning?
must he be underpinning?
must we be underpinning?
must you be underpinning?
must they be underpinning? 	must I have underpinned?
must you have underpinned?
must he have underpinned?
must we have underpinned?
must you have underpinned?
must they have underpinned? 	must I have been underpinning?
must you have been underpinning?
must he have been underpinning?
must we have been underpinning?
must you have been underpinning?
must they have been underpinning? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
mustn't I underpin?
mustn't you underpin?
mustn't he underpin?
mustn't we underpin?
mustn't you underpin?
mustn't they underpin? 	mustn't I be underpinning?
mustn't you be underpinning?
mustn't he be underpinning?
mustn't we be underpinning?
mustn't you be underpinning?
mustn't they be underpinning? 	mustn't I have underpinned?
mustn't you have underpinned?
mustn't he have underpinned?
mustn't we have underpinned?
mustn't you have underpinned?
mustn't they have underpinned? 	mustn't I have been underpinning?
mustn't you have been underpinning?
mustn't he have been underpinning?
mustn't we have been underpinning?
mustn't you have been underpinning?
mustn't they have been underpinning?