Given a regular Dirichlet form $(\mathcal{E},\mathcal{F})$ on a fixed domain $E$ of $\mathbb{R}^d$, we first indicate that the basic assumption $C_c^\infty(E)\subset \mathcal{F}$ is equivalent to the fact that each coordinate function $f^i(x)=x_i$ locally belongs to $\mathcal{F}$ for $1\leq i\leq d$. Our research starts from these two different viewpoints. On one hand, we shall explore when $C_c^\infty(E)$ is a special standard core of $\mathcal{F}$ and give some useful characterizations. On the other hand, we shall describe the Fukushima's decompositions of $(\mathcal{E},\mathcal{F})$ with respect to the coordinates functions, especially discuss when their martingale part is a standard Brownian motion and what we can say about their zero energy part. Finally, when we put these two kinds of discussions together, an interesting class of stochastic differential equations are raised. They have uncountable solutions that do not depend on the initial condition.