teacher please help - must have been

[Unregistered]

John said there was not much traffic on the road that day but one driver suddenly appeared and he struck that vehicle. He believes that driver must have been going very fast.

thank you

[billmcd]

"Must have been" is used in your example as a conclusion based on some perception or real or imagined evidence.

[bertietheblue]

must be driving very fast = based on the evidence, I think he is driving very fast - PRESENT USAGE, CONTINOUS FORM ('I am doing')
must have been driving very fast = based on the evidence, I think he was driving very fast - PAST USAGE, CONTINOUS FORM ('I was doing')