(no subject)

[clownshoes.livejournal.com]

My described three phrases to mean "before":

Перед тем как
Прежде чем
До того как

But it does not tell me when to use each one. In my workbook, we have exercises where it asks for particular ones. Example: "__________ пойти спать, он подошел к бабушке и попросил ч нее прощения." It gives the answer as Перед тем как, but I don't understand why that is right, as opposed to the others.