この鍵で彼の部屋に入れます。

this (something or someone close to the speaker (including the speaker), or ideas expressed by the speaker)

key; lock

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.

he; him; his; boyfriend

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

room; apartment; flat; pad; chamber; stable

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 put in; to let in; to take in; to bring in; to insert; to set (a jewel, etc.); to ink in (e.g. a tattoo); to admit; to accept; to employ; to hire; to accept; to comply; to grant; to adopt (a policy, etc.); to take (advice, etc.); to listen to; to pay attention to; to include; to pay (one's rent, etc.); to cast (a vote); to make (tea, coffee, etc.); to turn on (a switch, etc.); to send (a fax); to call