Essentially, Federal rules force American firms to pay U.S. taxes on their foreign operations in situations where competitors based abroad do not pay tax on their similar operations.

基本上，联邦法律会要求美国公司就其海外业务向美国政府交税，条件是海外的竞争者不就类似的业务交税。
主语是Federal rules，谓语是force 宾语是American Firms。
in situation where competitors based abroad do not pay tax on their similar operations是修饰foerign operations的，表示一种条件，条件就是where competitors based abroad do not pay tax on their similar operations, 即：海外的竞争者不为类似的业务交税。