THe Differential of a Function is a One Form

Let  
\[\mathbf{a} =(a_1 , a_2 , a_3) \in \mathbb{R}^3\]
  be a vector.
\[\begin{equation} \begin{aligned} d_{\mathbf{x}} (\mathbf{a}) &= \frac{\partial}{\partial x_1} (\mathbf{x})a_1 + \frac{\partial}{\partial x_2} (\mathbf{x})a_2 + \frac{\partial}{\partial x_3} (\mathbf{x})a_3 \\ &= \frac{\partial}{\partial x_1} (\mathbf{x})dx_1 (\mathbf{a}) + \frac{\partial}{\partial x_2} (\mathbf{x})dx_2 (\mathbf{a}) + \frac{\partial}{\partial x_3} (\mathbf{x})dx_3 (\mathbf{a}) \end{aligned} \end{equation}\]

The coefficient functions are the  
\[\frac{\partial}{\partial x_2} (\mathbf{x})\]
.
This shows that the differential of a function is a one form.