Forsythias grow so fast once they establish that you probably don't have to do much right now. As it sends up new growth from the ground level, you can prune out some of the branches that you don't want....just a few at a time. We had a cement truck drive over ours, but 3 years later, it looks pretty good again. If there are lots of reserves in the roots, it can send up a 6 or 8 foot shoot from the soil line in one year. If it's younger, new growth will be smaller.

New plants should be given 2 or three years to establish a decent framework, doing light pruning in the spring, after flowering. If it needs renovation, this is best done over two years, pruning in late winter or early spring--never in the fall.