Lao Mao, please help!

Lao Mao,

I sent out the application of transfer from H1B to F1 10 days before my H1B expired. Now 10 days passed and I have not received the recepit from USCIS. Right now I do not know if I am still legal to stay in America. Am I still legal? or do I have to leave America now? Can I think my status is pending even if I have not received the recepit? I have confired that USCIS has received my express mail 10 days ago.

Thank you in advance!