Line 2, hemlock: a poison made from an herb or a poisonous drink made from that herb.
Line 4, Lethe: a river in Hades (the underworld). Souls about to be reincarnated drank from it to forget their past lives.
Line 7, Dryad: a wood nymph or nymph of the trees. Dryads or nymphs were female personifications of natural features, like mountains and rivers; they were young, beautiful, long-lived and liked music and dance. A Dryad was connected to a specific tree and died when the tree died.

Line 3, Flora: goddess of flowers and fertility.
Line 4. Provencal: of Provence, an area in the south of France associated with song, pleasure, and luxury.
Line 6, Hippocrene: a spring sacred to the Muses, located on Mt.Helicon. Drinking its waters inspired poets. (The nine muses were associated with different arts, such as epic poetry, sacred song, and dancing.)

Line 2, Bacchus: Roman god of wine.
            pards: leopards, which drew Bacchus's chariot.
Line 3, viewless: invisible.
            Poesy: poetry in general or, depending n how you read this ode,             a specific kind of poetry--visionary poetry or fantasy.
Line 6, haply: perhaps, by chance.
Line 7, fays: fairies.
Line 10, verdurous: green.

Line 3, embalmed: (1) fragrant, (2) preserved body. Is Keats using both meanings here to suggest the inextricably mixed nature of life?

Line 1, darkling: in the dark.
Line 10, requiem: song or musical service for the repose of the dead.

Line 6, Ruth: Boaz saw Ruth, the Moabite, working in the fields, fell in love with her and married her; David is one of her descendents. A book in the Bible is named after her. She is frequently alluded to by poets for her devotion to her mother-in-law Naomi or as a stranger in a strange land. In a sense she has achieved immortality.
Line 7, corn: grain, often wheat, in British usage.
Line 9, casements: windows.

Line 5, plaintive: expressing sadness.
            anthem: (1)a hymn of joy or praise, patriotism, or devotion; (2) a sacred choral song generally based on words from the Bible. Both meanings carry with them intense feelings and high seriousness. Which meaning do you think Keats intends? Does one fit better with requiem? Can both meanings be meant or suggested?