Should I split firewood when it's wet or dry?
Is it easier to split green wood or dry?
As a general rule, it's easier to split wood when it is fresh or green. That being said, most wood isn't going to dry unless it is split. Unless you live in a dry climate and cut the wood to short lengths, it's doubtful that you'll ever split completely dry wood. I consider there to be two types of firewood: wood that has been processed and seasoned properly, and wood that is eventually going to rot on the ground.
The bottom line is that the quicker the wood is split, the quicker it will season. For that reason, split your firewood as quickly as you can to prepare it for burning.
Hope this helps!