Complex sentence: A monkey that begins its move in the river might leave and return to the river once or more during a multi-capture move; in that case the monkey drowns only if the multi-capture move ends in the river.
Split into simpler sentences: A monkey that begins its move in the river might leave and return to the river once or more during a multi-capture move. In that case the monkey drowns only if the multi-capture move ends in the river.