回复：多谢 863211 - 再请指点

1. H-1B1 visa US Singapore FTA (like TN), expired Sep 10 2009 - so my unlawful presence will be 3 month by Dec 10 2009?
-No, it is counted from the date (1/20/2010) that USCIS request you to leave after your F1 application was denial.

2. I suspect denial was due to OOS after layoff before filing for I-539 - RFE was evidence on payroll between 7/20/09 to 8/1/09. I will update once I get the letter.
-Then, it is most likely that reason.

3. I probably will need to go back to Singapore to apply for F-1. How would my OOS affect my F-1 application?
-You just go to Singapore to apply for the F1 visa and I believe you should have no problem to get it.