Brief analysis:
"The lonely sail is far away, and the sky is exhausted." Li Bai has been sending his friends to the boat, and the boat has already left. He is still watching the sails in the distance by the river. Li Bai's eyes looked at the sail shadow until it gradually blurred and disappeared at the end of the blue sky, showing a long time to watch. The shadow of the sail has disappeared, but Li Bai still stared at it, and then he noticed that a river with spring water was flowing to the distant junction of water and sky.
"Just watching the Yangtze River flowing in the sky" is the scene in front of us, not just writing about the scenery. Li Bai's deep affection and yearning for his friends are all reflected in this poetic style. The poet's heart rises and falls, just like a stream of spring water rolling eastward.