How do you know when to use justified trailing or leading

If they give us 07 data and 08 data, and its currently 08. To calc leading P/E, is 08 data d1 (and we dont include grwth rate) and for trailing P/e do we use 07 Do x growth rate (or 08 data with growth rate) Or better yet, in one question we were just given this years data, and they used trailing P/E, so basically when do you know when to use trailing or forwad P/e and what do you use for Do?

D0 remains as D0. Trailing uses E0, whereas leading uses E1 Leading P/Es are usually used for forecasting P/E

I need more of an explaination

The question will specify which ratio they want you to calculate.