He fell down in the mountain.
Kare wa sanchū de taoreta.
he; him; his; boyfriend
topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis
in the mountains; among the mountains
indicates location of action; at; in; indicates time of action; indicates means of action; cause of effect; by; and then; so; indicates continuing action; indicates certainty, emphasis, etc.
to fall; to collapse; to drop; to fall senseless; to be forced to bed (by illness, etc.); to die; to go bankrupt; to be ruined; to have a bad debt; to be defeated (in a game); to fall (of governments, dictators, etc.)