あいにくご招待をお受けできないのですが。

unfortunately; Sorry, but ...

honorific/polite/humble prefix; honorific suffix

invitation

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.

honorific/polite/humble prefix

to receive; to get; to catch (e.g. a ball); to be struck by (wind, waves, sunlight, etc.); to sustain (damage); to incur (a loss); to suffer (an injury); to feel (influence); to undergo (e.g. surgery); to take (a test); to accept (a challenge); to be given (e.g. life, talent); to follow; to succeed; to be descended from; to face (south, etc.); to be modified by; to obtain (a pawned item, etc.) by paying a fee; to be well-received; to become popular; to go down well

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

be; is

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and