There are three levels of meaning in this passage as follows:

1. I'm shelling my crab, I'm writing in my notebook, and the sky is full of me falling on maple leaves and snowflakes, while I'm thinking of you.

2, The crab can't be shelling me, the notebook can't be writing about me, the sky can't be roaming with me falling on maple leaves and snowflakes, and you can't be like me.

3. Unless the world is upside down, you're thinking of me.

From the first place of Wuhan University's three quotes, meaning that only if the world is upside down you will fall in love with me.

Expanded

Appreciation:

Only when the world is upside down will you fall in love with me, and can be read all the way through, I'm peeling the shells of the crabs, I'm writing in my notebook, and I'm thinking about you while the sky is full of snowflakes and maple leaves falling on me. This is typical unrequited love , expressing the author's helplessness to the real situation. All these are impossible, the crab can't peel my shell, the notebook can't write about me, I can't fall on the maple leaves and snowflakes, you can't be thinking about me.

Rambling I fall on maple leaves and snowflakes, the romantic atmosphere of this sentence began to dense out, the colors in the verse in this piece also began to reflect the warmth of the red and cold white, as if passionate love meets relentless indifference.

The "diffuse sky" created by the omnipresent is like a longing, hope to sow around the person, all the time around her (his) side, but also like in the cluttered and disorderly picture reveals a trace of anguish.