@paraoprao , once your visa status is adjusted to work permit visa ( H-1..) you start counting days present in the USA for passing the Substantial Presence Test (SPT). Till you pass that test you are considered a Non-Resident Alien ( NRA) and thereafter a Resident for Tax Purposes.
During NRA period, you are taxed ONLY on US sourced/connected income , , file your return on form 1040-NR ( not supported by TurboTax, but by SprinTax or similar and/or tax professional), and subject to FICA taxes.
Once you pass the SPT ( 183 days present counting all days present in the Tax Year, PLUS 1/3 rd. the days present in the 1st. previous yea, PLUS 1/6th the days in the 2nd previous year ), you are now taxed on your world income and file on form 1040. There are other ifs, buts etc. but this is main diff. between NRA and "Resident for Tax Purposes" taxation .
As you will note from above FICA is levied on all income ( unless you are part of the safe haven / exclusion band such as OPT, CPT etc. )
See here --> Employers Must Withhold FICA Taxes for Aliens who Change Visa Status to H1B | Internal Revenue Servi...
Does this answer your query ?
