A proton moving in a straight line enters a strong magnetic field along the field direction then its path and velocity will be

$F=q(\vec{V}\times \vec{B})$
$=qVBsin0$
$=qVBsin{0}^0$        $(\because \text{V and B has same direction})$
$=0$                         $(\because sin{0}^0=0)$
So, there is no force, so, path is same and velocity is constant.