Complex sentence: It officially began May 15,1988 in the eastern Pacific, and June 1,1988 in the central Pacific and lasted until November 30,1988, dates that conventionally delimit the period of each year when most tropical cyclones form in the northeastern Pacific Ocean.
Split into simpler sentences: It officially began May 15,1988 in the eastern Pacific, and June 1,1988 in the central Pacific and lasted until November 30,1988. These dates conventionally delimit the period of each year when most tropical cyclones form in the northeastern Pacific Ocean.