日常对话中用单数，文学描写中用复数。
(1) Sky 天空
There isn't a cloud in the sky!
天空万里无云！
The sky looks very ominous with all those dark clouds.
天空乌云密布，看起来不妙。
(2) Skies 天空
The pilot took to the skies, the place he felt most free from the worries of life below.
飞行员飞向天空，那是他觉得最自由、可以远离地面生活烦恼的地方。
The skies looked like a watercolor painting with vibrant pinks, oranges, and yellows blending together as the sun sank closer to the horizon.
随着暮日沉向地平线，天空像一幅水彩画，热烈纷繁的粉、橙、黄色交织在一起。
