"Love without hope"
This is the whole idea of the poem, bringing with it an introductory state and function here to express at first glance what the poem is about.
"So let the imprisoned larks escape and fly"
This is perhaps the most powerful line of the poem, showing at last that the love between the bird-catcher and the squire's daughter has aligned. The quote's meaning is that the characters are finally free, like the birds that the bird-catcher once valued above other things. This is perhaps the most influential line of the poem, and the one that gives the main idea a whole new meaning.
