When all at once I saw a crowd, A host, of golden daffodils; (lines 2-3)
You don’t think about verb tenses through much of the poem. It reads like a straightforward narrative which, though not set in the present, still has this present-y atmosphere to it because of phrases like "all at once."
A poet could not but be gay, In such a jocund company: (lines 13-14)
The poem could have worked just fine if it ended here. Sad man becomes
happy: a perfect plot. But no, Wordsworth has to be Wordsworth and add
more layers of complexity.
For oft, when on my couch I lie In vacant or in pensive mood, (lines 19-20)
The final stanza hits us with a surprise: the daffodil scene wasn’t
something that happened yesterday, or even last week. It happened a
while ago. Now the speaker is giving us a memory about his memories.
They flash upon that inward eye Which is the bliss of solitude; (lines 21-22)
The daffodils have become a memory, but one that returns with full,
vivid force at strange moments, like when the speaker is just sitting
around, bored, with nothing in his head. It’s as if the images of
daffodils fill an empty space where thoughts should be ("vacant"), or
they push out other thoughts when there are too many ("pensive").
And then my heart with pleasure fills, And dances with the daffodils. (lines 23-24)
The spontaneous and unexpected return of the daffodils to his "inner
eye" brings the speaker back to the day he first saw them. Music and
dancing are the ultimate expressions of joy in the present moment, but
he can rediscover these joys even in his memory.