Modeling derivational morphology to generate words with particular semantics is useful in many text generation tasks, such as machine translation or abstractive question answering. In this work, we tackle the task of derived word generation. That is, given the word "run," we attempt to generate the word "runner" for "someone who runs." We identify two key problems in generating derived words from root words and transformations: suffix ambiguity and orthographic irregularity. We contribute a novel aggregation model of derived word generation that learns derivational transformations both as orthographic functions using sequence-to-sequence models and as functions in distributional word embedding space. Our best open-vocabulary model, which can generate novel words, and our best closed-vocabulary model, show 22% and 37% relative error reductions over current state-of-the-art systems on the same dataset.