手紙を出すのを忘れないでね。

letter

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 ...

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)

indicates emphasis, agreement, request for confirmation, etc.; is it so; hey; come on; listen; not