彼らは流れに乗って川を下った。

they (usually male); them

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

stream; current; flow; course of events

at (place, time); in; on; during; to (direction, state); toward; into; for (purpose); because of (reason); for; with; by; from; as (i.e. in the role of); per; in; for; a (e.g. "once a month"); and; in addition to; if; although

to get on (train, plane, bus, ship, etc.); to get in; to board; to take; to embark; to get on (e.g. a footstool); to step on; to jump on; to sit on; to mount; to reach; to go over; to pass; to follow; to stay (on track); to go with (the times, etc.); to take part; to participate; to join; to get into the swing (and sing, dance, etc.); to be deceived; to be taken in; to be carried; to be spread; to be scattered; to stick; to attach; to take; to go on

river; stream; the .... river; (suffix used with the names of rivers)

indicates direct object of action; indicates subject of causative expression; indicates an area traversed; indicates time (period) over which action takes place; indicates point of departure or separation of action; indicates object of desire, like, hate, etc.

to come down; to go down; to fall; to drop; to sink; to get lower; to hang; to dangle; to move back; to step back; to withdraw; to retire; to deteriorate; to fall off; to be downgraded; to get closer to the present day; to go south