conductor $A$is given a charge $+Q$and then placed inside a deep metal can $B$, without touching it. Then

When A is placed inside the $B$, net charge on the can  will be zero and potential will not change.
For earthing the charge will flow from higher potential to lower potential.
When $B$ is earthed, the charge $Q$ flows from the outer surface  into the earth and induced charge -Q resides on the inner surface of the can, as a result the potential of $A$is reduced.