学校のオーケストラに入るつもりなのだ。

school

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

orchestra

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 in; to go in; to come in; to flow into; to set; to set in

intention; plan; conviction; belief

be; is; indicates past or completed action; indicates light imperative