Retrograde motion happens when earth overtakes an outer planet in orbit. Being closer to the sun than the outer planets earth moves along faster and will pass between the sun and the outer planet at least once a year. This is what causes retrograde motion as seen from earth.
The inner planets, Mercury and Venus move along in their orbits faster than earth and so earth is never overtaking them, per se. The time that we would see retrograde behavior in the inner planets is when they are eclipsed by the sun, so we never see them retrograde.
Planets show a little sign of phase change Outer planets are more difficult to notice a phase change because we are always between the outer planet (whichever one) and the sun, so we always see it as nearly "full" The exception would be when earth, the sun, and the outer planet make a right angle in their positions. At this time you would see the smallest gibbous of the planet (it would look like a small sliver of the night side) Mars, being closer to us, is more noticeable when lined up just right.
The inner planets, in contrast, are in a "new phase" when they are closest to us. This is because they are closest to earth when they are between earth and the sun. As they move ahead of us in orbit we see a crescent phase turn into a quarter phase, but by the time they are in quarter phase the earth, sun, and inner planet make a right angle, meaning the planet is farther away. As it progresses to "full phase" it is being eclipsed by the sun and twice as far away, if not more, than it was at "new phase" So we can never see the inner planets completely full.