I have a backache.
Senaka ga itai no desu.
back (of body)
indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and
painful; sore; exceeding
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