  This report introduces two recent measurements of semileptonic $b$-hadron
decays at the LHCb experiment, including a test of Lepton Flavour Universality
(LFU) using $\bar{B}^0 \to D^{(*)+} l^- \bar{\nu}_l$ decays where
$l\in\{\mu,\tau\}$, and a study of the $D^{*+}$ longitudinal polarisation in
the $\bar{B}^0 \to D^{*+} \tau^- \bar{\nu}_\tau$ decay. With the inclusion of
the new results of the LFU ratios, the world average on $R(D)$ and $R(D^*)$,
still shows a tension over three standard deviations from the SM prediction,
while the measured $D^{*+}$ longitudinal polarisation is compatible with the SM
value.
