uniform.
She must be a doctor
(=I am sure she is a doctor).
She was wearing a medical uniform.
She must have been a doctor
(=I am sure she was a doctor).
Might, may, could = you think smth is possible
She is wearing a medical uniform.
She might be a doctor
(=I am not 100% sure but it’s possible that she is a doctor).
She was wearing a medical uniform.
She might have been a doctor
(=I am not 100% sure but it’s possible that she was a doctor).
CAN’T, COULDN’T = you are sure smth is/was not true
She doesn’t know medicine.
She can’t/couldn’t be a doctor
(=I am sure she isn’t a doctor).
She didn’t know medicine.
She can’t/couldn’t have been a doctor
(=I am sure she wasn’t a doctor).