BG1Chapter 1, escaping from Candlekeep.
Chapter 2, outside but not yet in Naskel.
Chapter 3, after going to Naskel, but before looting the chest in the Naskel Mines.
Chapter 4, after that and before looting the Bandit Camps chest.
Chapter 5, killing Davaeorn in the Cloakwood Mines should start the next chapter.
Chapter 6, getting to talking to the grand Duke Elthan and taking to book.
Chapter 7, after facing Sarevok first time.
Chapter 8, the chapter changes after talking to Belt the last time
BG2:
SoAChapter 1, Irenicus Dungeon
Chapter 2, after you have heard the Shadow Thieves deal.
Chapter 3, after getting the money and doing a deal with either party of the conflict.
Chapter 4, Brynnlaw, Asylum, Sahuagin City
Chapter 5, Underdark.
Chapter 6, Out of the Underdark, hunt Lanthorn.
Chapter 7, Suldanessellar.
Chapter 8, starts killing Irenicus the second time.
ToBChapter 8
ToB until Oasis
Chapter 9 Sendai, Abazigal, Balthazar, Throne of Bhaal: DEFEAT AMELYSSAN