In this article we deal with the algebra of generalized Hukuhara differentiable fuzzy functions (gH-differentiable fuzzy functions, for short). We show that the sum and the gH-difference of two gH-differentiable fuzzy functions are not necessarily gH-differentiable. Then we give conditions on the two fuzzy functions so that the sum and the gH-difference of these to be gH-differentiable. Moreover we present practical rules for obtaining the gH-derivative of these fuzzy functions. Also, we give a rule for obtaining the gH-derivative of fuzzy functions generate from the multiplication of a real number by a gH-differentiable fuzzy function. The results presented here correct the results obtained previously by other authors.