Fall down and Fall in

Fall down verb - Lose an upright position suddenly.

Fall in and fall down are semantically related in collapse inward topic. In some cases you can use "Fall in" instead a verb "Fall down".

Fall in verb - Break down, literally or metaphorically.

Fall down and fall in are semantically related in collapse topic. You can use "Fall down" instead a verb "Fall in", if it concerns topics such as collapse inward.