We're cheating a little bit here. See, nature is also part of Keats' whole understanding of negative capability: looking at a huge, scary mountain or the tumult of a stormy sea are a way to face big, mysterious things that are completely outside of the viewer's control. In other words, they're a constant reminder of all that we can't understand. Looking at nature is like a shortcut to Keats' "negative capability."
Line 4: Language is compared to wheat in a grain bin this <strong>simile</strong> (remember, similes use "like" or "as" in their comparison of two things).
Lines 5-6: Night and clouds become <strong>symbols</strong> of love in this passage. Oh, it's all too heavenly to believe – and maybe that's the point.
Line 9: The "fair creature" becomes an <strong>image</strong> of the natural world, but she's an especially weak one. Unlike the sky or the world, she'll grow old and pass away.
Line 13: The "wide world" becomes <strong>an image</strong> of nature in general – a world far too big to understand.