$(i)$ When no field is present $E =0, B =0$, the proton experiences no force. Thus it moves with a constant velocity.

$(ii)$ When $E=0$ and $B \ne 0$, then there will be a probability that proton may move parallel to magnetic field. In this situation, there will be no force acting on proton.

$(iii)$ When both fields are present

$E \ne 0, B \ne 0$, then let $E , B$ and $v$ may be mutually perpendicular to each other.

In this case, the electric and magnetic forces acting on the proton may be equal and opposite. Thus, there will be no resultant force on the proton.