Question

Why are there two high tides a day about 12 hours apart from each other?

Earth revolves around the Sun once during that time.
Earth rotates entirely on its axis once during that time.
The moon revolves around Earth once during that time.
The moon’s orbit is in line with Earth’s orbit twice a day.

Answer 1

Because as Earth spins once each day a given location passes beneath two tidal bulges: one on the side facing the Moon (stronger lunar gravity) and one on the opposite side (resulting from the system's centripetal/centrifugal balance). That rotation carries you through those two bulges roughly 12 hours apart. (In reality the interval is about 12 h 25 min because the Moon moves in its orbit.)