Tā zài chuāng shàng tiē le yī zhāng hóngsè jiǎnzhǐ.
She stuck a red papercut on the window.
Tips
grammar
Not a compound word — 窗 (window) + 上 (locative 'on/at'). The pattern noun + 上 marks a surface or location: 桌子上 (on the table), 墙上 (on the wall), 街上 (on the street). The 上 here is read in the neutral/light tone (shang) in fluent speech.
