What's going on, basically, is an abuse of notation in which ##\frac{1}{1-\frac{\partial}{\partial t}}## is a shorthand for the inverse operator ##L^{-1}= \left(1-\frac{\partial}{\partial t}\right)^{-1}##, where ##L = 1 - \frac{\partial}{\partial t}##.
Under certain conditions, you can write...