I can’t emphasize this last point enough. If you are not married, and your partner puts money in your bank account, that is a gift to you. It becomes your money forever, and your partner no longer has any rights to that money. Any interest earned by the money belongs to you and you are solely responsible for the taxes. Presumably, you plan to give the money back to your partner, and give your partner some or all of the interest. But there would be no legal requirement that you do so. This is more of a warning to your partner, that if you were to have a bad break up, your partner might have great difficulty in getting the money back if you were not willing to do it voluntarily.
Also, be aware that if you were to try to get around the gift problem by declaring that this money is a loan, the IRS will require that you pay interest to your partner at the federally required minimum rate or higher. The federal minimum rate is variable and is currently around 6%. Your partner would be required to report taxable income equal to the interest that you should have paid using the federal minimum rate, even if you didn’t actually pay interest. That creates a situation where you receive interest from the bank, that you pay tax on, and then you must pay interest to your partner that your partner pays tax on.
