could investigate
Regular verb: investigate - investigated- investigated

Affirmation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could investigate
you could investigate
he could investigate
we could investigate
you could investigate
they could investigate 	I could be investigating
you could be investigating
he could be investigating
we could be investigating
you could be investigating
they could be investigating	I could have investigated
you could have investigated
he could have investigated
we could have investigated
you could have investigated
they could have investigated	I could have been investigating
you could have been investigating
he could have been investigating
we could have been investigating
you could have been investigating
they could have been investigating	

Negation 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
I could not investigate
you could not investigate
he could not investigate
we could not investigate
you could not investigate
they could not investigate 	I could not be investigating
you could not be investigating
he could not be investigating
we could not be investigating
you could not be investigating
they could not be investigating	I could not have investigated
you could not have investigated
he could not have investigated
we could not have investigated
you could not have investigated
they could not have investigated	I could not have been investigating
you could not have been investigating
he could not have been investigating
we could not have been investigating
you could not have been investigating
they could not have been investigating	

Interrogative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I investigate?
could you investigate?
could he investigate?
could we investigate?
could you investigate?
could they investigate? 	could I be investigating?
could you be investigating?
could he be investigating?
could we be investigating?
could you be investigating?
could they be investigating? 	could I have investigated?
could you have investigated?
could he have investigated?
could we have investigated?
could you have investigated?
could they have investigated? 	could I have been investigating?
could you have been investigating?
could he have been investigating?
could we have been investigating?
could you have been investigating?
could they have been investigating? 	

Interro-negative form 	
Simple form 	V-ing form 	Perfect 	V-ing perfect 	
could I not investigate?
could you not investigate?
could he not investigate?
could we not investigate?
could you not investigate?
could they not investigate? 	could I not be investigating?
could you not be investigating?
could he not be investigating?
could we not be investigating?
could you not be investigating?
could they not be investigating? 	could I not have investigated?
could you not have investigated?
could he not have investigated?
could we not have investigated?
could you not have investigated?
could they not have investigated? 	could I not have been investigating?
could you not have been investigating?
could he not have been investigating?
could we not have been investigating?
could you not have been investigating?
could they not have been investigating?