Question

when we see mars going through a period of apparent retrograde motion, it means mars is temporarily moving backward in its orbit of the sun mars & earth must be on opposite sides of the sun earth is passing mars in its orbit (both planets are on the same side of the sun) mars looks dimmer than usual

Explanation:

Apparent retrograde motion of Mars occurs when Earth, moving faster in its orbit, passes Mars while both are on the same side of the Sun. This makes Mars seem to move backward in the sky from our perspective on Earth. Mars is not actually moving backward in its orbit. When they are on opposite sides of the Sun, no retrograde motion is seen. And retrograde motion has nothing to do with Mars's brightness.

Answer:

Earth is passing Mars in its orbit (both planets are on the same side of the Sun)