彼の手紙に返事をださなきゃ。

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

letter

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

reply; answer; response

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 out; to get out; to put out; to reveal; to show; to submit (e.g. thesis); to turn in; to publish; to make public; to send (e.g. letter); to produce (a sound); to start (fire); to serve (food); to begin ...; to start to ...; to burst into ...