must yak
Regular verb: yak - yaked- yaked

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I must yak
you must yak
she must yak
we must yak
you must yak
they must yak 	I must be yaking
you must be yaking
she must be yaking
we must be yaking
you must be yaking
they must be yaking	I must have yaked
you must have yaked
she must have yaked
we must have yaked
you must have yaked
they must have yaked	I must have been yaking
you must have been yaking
she must have been yaking
we must have been yaking
you must have been yaking
they must have been yaking	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I mustn't yak
you mustn't yak
she mustn't yak
we mustn't yak
you mustn't yak
they mustn't yak 	I mustn't be yaking
you mustn't be yaking
she mustn't be yaking
we mustn't be yaking
you mustn't be yaking
they mustn't be yaking	I mustn't have yaked
you mustn't have yaked
she mustn't have yaked
we mustn't have yaked
you mustn't have yaked
they mustn't have yaked	I mustn't have been yaking
you mustn't have been yaking
she mustn't have been yaking
we mustn't have been yaking
you mustn't have been yaking
they mustn't have been yaking	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
must I yak?
must you yak?
must she yak?
must we yak?
must you yak?
must they yak? 	must I be yaking?
must you be yaking?
must she be yaking?
must we be yaking?
must you be yaking?
must they be yaking? 	must I have yaked?
must you have yaked?
must she have yaked?
must we have yaked?
must you have yaked?
must they have yaked? 	must I have been yaking?
must you have been yaking?
must she have been yaking?
must we have been yaking?
must you have been yaking?
must they have been yaking? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
mustn't I yak?
mustn't you yak?
mustn't she yak?
mustn't we yak?
mustn't you yak?
mustn't they yak? 	mustn't I be yaking?
mustn't you be yaking?
mustn't she be yaking?
mustn't we be yaking?
mustn't you be yaking?
mustn't they be yaking? 	mustn't I have yaked?
mustn't you have yaked?
mustn't she have yaked?
mustn't we have yaked?
mustn't you have yaked?
mustn't they have yaked? 	mustn't I have been yaking?
mustn't you have been yaking?
mustn't she have been yaking?
mustn't we have been yaking?
mustn't you have been yaking?
mustn't they have been yaking?