彼らは錠を下ろすのを忘れた。

they (usually male); them

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

lock; padlock; tablet; lozenge; pill

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 take down (e.g. flag); to launch (e.g. boat); to drop; to lower (e.g. ladder); to let (a person) off; to unload; to discharge; to drop off (a passenger from a vehicle); to let (a person) off; to withdraw money from an account; to wear (clothing) for the first time; to fillet (e.g. a fish)

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

to forget; to leave carelessly; to be forgetful of; to forget about; to forget (an article)