As an immigration lawyer, I often come across this problem. Someone enters the U.S. with a B-1/B-2 visitor visa. At the airport, they are given a 6 month stay in the U.S. But they want to stay in the U.S. longer than that. So they go on the internet, come across Form I-539, Application to Extend Stay. Without consulting with an immigration lawyer, they fill out the I-539 themselves and file it with USCIS. The I-539s are routinely denied and then the person is considered to be out of status and to have accrued unlawful presence. As a result, they are unable to get visas or green cards in the future all because they foolishly filed Form I-539.
You should expect your I-539 to be denied unless it is meticulously well-documented. Your application will only be approved if you submit extensive documentation to show that you have strong ties to outside the United States. You should show, for instance, that you have a spouse or child in your home country or that you have a job in your home country and own real estate there. You should also show that you have adequate money to support yourself during your requested stay.
If you can’t come up with this documentation, and particularly if you have already been in the United States for an extended period of time, USCIS may not be convinced that you are truly planning to depart the U.S. at the end of your temporary stay as a visitor.
In my experience as an immigration lawyer, filing an I-539 to try to extend your stay as a visitor in the US is a terrible idea in most cases. Don’t file an I-539 without consulting with a immigration lawyer first.