wǒ bù zhī dào shì shéi de ? I don´t know whose is this.
wǒ gěi nǐ . – I give it to you.
zhè gè bú shì wǒ de . This is not mine.
zhè gè shì nǐ de ? Is it yours?
bú shì nǐ de? Not yours?
nà shì shéi de? Then, whose is this?
wǒ zhī dào . zhè shì Mary de. I know, this is Mary's.
Attention: the Pinyin for "it" is zhè, not zhèi as in the video.
I think three points make this video very good: 1. At the beginning of the video there's always a review of the last lesson. 2. They make the video interactive and live. The examples in the video are also good. 3. They have subtitles in both English and Spanish
This video (see at YouTube) is of medium level difficulty. It has subtitles in English, Pinyin and Chinese characters. We especially recommend this video.