3 sentences

1. 天冷了，你穿得太单薄了。

   tiān lěng le, nǐ chuān de tài dān​bó le.

   It's cold; you're dressed too thinly.

   Source: Dong Chinese Dictionary

2. 那个孩子从小身体单薄，经常生病。

   nà​ge hái​zi cóng xiǎo shēn​tǐ dān​bó, jīng​cháng shēng​bìng.

   That child has been frail since childhood and often gets sick.

   Source: Dong Chinese Dictionary

3. 这部小说的人物形象塑造得比较单薄。

   zhè bù xiǎo​shuō de rén​wù xíng​xiàng sù​zào de bǐ​jiào dān​bó.

   The character development in this novel is rather thin.

   Source: Dong Chinese Dictionary