I have few questions about filing taxes. In the year 2010 I was on OPT(F1) from Jan 1,2010 - Dec 8,2010 and from Dec 9,2010 - Current date my status changed to H1b. Now, my question is on which status I have to file taxes on H1b or OPT(F1)?

I contacted one tax consultant and they informed me that if my status changes during a year from F1 to H1b I will be considered as Resident-Alien and not as non-Resident-Alien(as on F1) and I end up owing money to IRS. Am I considered on H1b for the whole year or how? I'm a little confused so I need your suggestions.

You will file as a non-resident alien, since number of days of your H-1B status was less than that required to pass the substantial presence test.

I came to USA in Aug 1,2006 and from Oct 6,2008 I am on OPT Status.As I mentioned for the year 2010 from Jan 1,2010 - Dec 8,2010 I am on OPT and from Dec 09,2010 my visa status is h1b. What is substantial presence test? How many days?

For 2010, you will file nonresident tax return and form 8843 and claim tax treaty deduction.