Complex sentence: Construction on the third planned stage, originally destined for the second half of the 1990s began in 1991, but because of constant offsets was extremely slow, and its first station opened in December 2002, and the second following five years later in 2007.
Split into simpler sentences: Construction on the third planned stage, originally destined for the second half of the 1990s began in 1991, was extremely slow because of constant offsets. Tts first station opened in December 2002, the second following five years later in 2007.