An inferior planet, closer to the Sun, will appear in phases, like the Moon.
As Mercury orbits, we generally only see it as a waxing or waning crescent.
At a superior conjunction, lined up Earth-Sun-planet, Mercury is full, but it usually can’t be seen from Earth because it’s behind the Sun.
We also cannot see it at inferior conjunction, lined up Earth-planet-Sun, because it gets lost in the light of the Sun.
Even if we could see it, it would look like a new moon, almost invisible.
Sometimes Mercury transits the Sun and is visible as a speck on the Sun’s surface.