Model why Moon phases occur due to viewing angle, not Earth's shadow, and distinguish the geometry and timing of solar versus lunar eclipses.
The Moon does not make its own light -- we see it because it reflects sunlight. At any moment, exactly half of the Moon is lit, just like half of Earth is in daylight at any moment. Phases happen because, as the Moon orbits, we see different amounts of that lit half from our angle on Earth -- not because Earth's shadow covers part of the Moon.
A solar eclipse happens when the Moon passes directly between the Sun and Earth, so the Moon's shadow falls on Earth -- only possible at new moon. A lunar eclipse happens when Earth passes directly between the Sun and the Moon, so Earth's shadow falls on the Moon -- only possible at full moon. Eclipses are not monthly because the Moon's orbit is tilted about 5 degrees compared to Earth's orbit, so the three bodies rarely line up exactly.
Mastery looks like: They model or explain why Moon phases occur due to viewing angle, and describe the alignment required for each eclipse type.
Common stumbles: Believing Earth's shadow causes the regular Moon phases; mixing up which eclipse happens at which phase.