What you learned in school
Physics textbooks taught that there were exactly four fundamental forces in nature: gravity, electromagnetism, strong nuclear force, and weak nuclear force. Students memorized these as the complete and final set of forces that explained all interactions in the universe. This "complete" list was presented as established fact.
What we know now
Physics textbooks taught four fundamental forces: gravity, electromagnetism, strong nuclear, and weak nuclear forces. Students memorized these as the complete set of forces in nature. Modern physics has shown that the electromagnetic and weak nuclear forces are actually aspects of a single electroweak force at high energies. Grand unified theories suggest that strong, weak, and electromagnetic forces may unify at even higher energies. String theory and other approaches propose that all forces, including gravity, may be manifestations of a single underlying force or principle.