When earth is between sun and moon why usually full moon but sometimes eclipse?

Answer 1

That is because the situation is complicated by the tilt of the Moon's orbit relative to the Earth's. That means that while the Sun stays always exactly on the ecliptic, the Moon goes above it and below on every 28-day circuit. That is why the Moon sometimes appears incredibly high or low in the sky. Usually at New Moon or Full Moon the Moon crosses north or south of the Sun, up to ten Sun-diameters above or below, apparently.

When the Moon is crossing the ecliptic going north (south) that position is called the ascending (descending) node. The Moon passes a node every two weeks.

If a New Moon (Full Moon) occurs when the Moon is at one of its nodes, there is a Solar (Lunar) eclipse.

The nodes rotate round the Moon's orbit in a cycle called the Saros cycle, just over 18 years (6585.3211 days), and every eclipse repeats itself after each Saros cycle. An eclipse is always part of a series, and the series starts with the Moon grazing the Sun to the north. After that the eclipse appears a bit further south at each recurrence, until it vanishes off the south.