The credit could be used to pay the debt to the IRS but leftover credit could not be used. The new law consolidates both credits and allows for the additional credit to be potentially refundable.
Your filing status and the tax rates that apply to your specific annual income will determine what bracket you fall into. The amount used to figure your taxes will be whatever income you have left after you have accounted any deductions you might be able to claim.
Tax deductions are subtracted from your income and what's left is subject to state and federal income taxes. They reduce the amount of income tax you would owe by decreasing your taxable income.