她是一个很单纯的女孩子。

Tā shì yīgè hěn dānchún de nǚ háizi.

She is a very naive girl.

他的动机很单纯，只是想帮助别人。

Tā de dòngjī hěn dānchún, zhǐshì xiǎng bāngzhù biérén.

His motives are pure; he just wants to help others.

我喜欢她单纯的笑容。

Wǒ xǐhuān tā dānchún de xiàoróng.

I like her innocent smile.