  Using a data sample corresponding to an integrated luminosity of
2.93~fb$^{-1}$ recorded by the BESIII detector at a center-of-mass energy of
$3.773$ GeV, we present an analysis of the decays $\bar{D}^0\to\pi^+\pi^0
e^-\bar{\nu}_e$ and $D^+\to\pi^-\pi^+ e^+\nu_e$. By performing a partial wave
analysis, the $\pi^+\pi^-$ $S$-wave contribution to $D^+\to\pi^-\pi^+ e^+\nu_e$
is observed to be $(25.7\pm1.6\pm1.1)$% with a statistical significance greater
than 10$\sigma$, besides the dominant $P$-wave contribution. This is the first
observation of the $S$-wave contribution. We measure the branching fractions
$\mathcal{B}(D^{0} \to \rho^- e^+ \nu_e) = (1.445\pm 0.058 \pm 0.039)
\times10^{-3}$, $\mathcal{B}(D^{+} \to \rho^0 e^+ \nu_e) = (1.860\pm 0.070 \pm
0.061) \times10^{-3}$, and $\mathcal{B}(D^{+} \to f_0(500) e^+ \nu_e,
f_0(500)\to\pi^+\pi^-) = (6.30\pm 0.43 \pm 0.32) \times10^{-4}$. An upper limit
of $\mathcal{B}(D^{+} \to f_0(980) e^+ \nu_e, f_0(980)\to\pi^+\pi^-) < 2.8
\times10^{-5}$ is set at the 90% confidence level. We also obtain the hadronic
form factor ratios of $D\to \rho e^+\nu_e$ at $q^{2}=0$ assuming the
single-pole dominance parameterization:
$r_{V}=\frac{V(0)}{A_{1}(0)}=1.695\pm0.083\pm0.051$,
$r_{2}=\frac{A_{2}(0)}{A_{1}(0)}=0.845\pm0.056\pm0.039$.
