Complex sentence: However, the longer a virtual particle exists, the more closely it adheres to the mass-shell relation, so a "virtual" particle that exists for an arbitrarily long time is simply an ordinary or "real" particle.
Split into simpler sentences: However, the longer a virtual particle exists, the more closely it adheres to the mass-shell relation. A "virtual" particle that exists for an arbitrarily long time is simply an ordinary or "real" particle.