Margaret, are you grievingOver Goldengrove unleaving? (1-2)
The poem opens with a question, but it's a question that the speaker seems already to know the answer to. (Don't you hate it when people do that?) It seems like a simple "yes" or "no" question, but it sparks all the musings of the rest of the poem. The answer to the question is, "Yes, she's grieving over the falling of the leaves in Goldengrove, but she's also grieving over something else."
Ah, as the heart grows olderIt will come to such sights colder (5-6)
The speaker sounds almost wistful when he sighs and says, "Ah" about Margaret growing less sensitive to the sad things in nature, like the falling of leaves in autumn. Isn't it a bummer to grow up?
And yet you will weep and know why. (9)
Even though she won't be as sensitive to nature when she gets older, Margaret will still "weep." The difference is that as an adult, she'll be able to put a name on the reason for her sorrow. So, maybe being a grown-up isn't so bad.