Can I "borrow" portions of a copyrighted song for one of my original songs?

Generally, under U.S. copyright law, copyright owners have the exclusive right to create derivative works, which is new work that uses all or a part of a pre-existing work. Before using a work created by another party, you should contact the owner and request a license. You may also want to consult with an attorney who is knowledgeable with U.S. copyright law.