Hello and thanks for your time.

I was on F1/OPT from May2009 to Oct-1st-2010.
On Oct 1st 2010 I changed to H1B. I went on vacation 21 days on December, so I was in the US for 70 days. That means as of right now I do NOT meet the Substantial Presence Test.

However, if I file an extension and I present my 2010 taxes on April 25th (after the 183 days), will I be able to file as a resident?

Can 2010 and 2011 days be combined for the 2010 Substantial Presence test?

Thanks.

Yes, you will combine days from 2010 and 2011.

Using the formula explained in IRS Pub 519, you would file sometime in June 2011.