Complex sentence: According to their own clocks, which appear to them to tick normally, they cross the event horizon after a finite time without noting any singular behaviour; it is impossible to determine the location of the event horizon from local observations.
Split into simpler sentences: According to their own clocks, which appear to them to tick normally, they cross the event horizon after a finite time without noting any singular behaviour. In particular, they are unable to determine exactly when they cross it, as it is impossible to distinguish the location of the event horizon from local observations.