Complex sentence: The Tenth Doctor is an incarnation of the Doctor, the protagonist of the BBC science fiction television programme "Doctor Who", who is played by Scottish actor David Tennant in three series as well as nine specials.
Split into simpler sentences: The Tenth Doctor is an incarnation of the Doctor, the protagonist of the BBC science fiction television programme "Doctor Who". He is played by Scottish actor David Tennant in three series as well as nine specials.