Simplify the following text for a Grade 9 native English-speaking person.
Original: Theta ( uppercase Î , lowercase Î or Ï ` ; Ancient Greek ; Modern Greek ; Ë Î iË tÉ , Ë Î eÉ tÉ ) is the eighth letter of the Greek alphabet , derived from the Phoenician letter Teth .
Simplified: Theta is the eighth Greek letter , written Î ( uppercase ) or Î ( lowercase ) .