I-485 pending; F1 expired in June 2009. AP one time used in January 2009 obtained I-94,expiration date is July 31,2009; EAD approved good from May 2009 to May 2011.
Questions:
1) It is illegal for me to stay in USA after July 31, 2009. (I-94)
-Your current status is AOS, not F1 since Jan. 2009 because you used AP. Yes, you can stay in U.S. legally with AOS until your I-485 approval/denial.
2) Can I stay in USA legally until May 2011? (EAD)
-Yes, see above answer. EAD is just for work.
3) Does I need to obtain a new I-94? and how to get it?
-Nothing needed to do.