  We present infrared spectroscopy of the 2019 eruption of the recurrent nova
V3890 Sgr, obtained over the period 5.1-46.3 days after the eruption. The
spectrum of the red giant became more prominent as the flux declined, and by
day 46.3 dominated the spectrum. Hydrogen and helium emission lines consisted
of a narrow component superposed on a broad pedestal. The full width at half
maximum of the narrow components declined with time $t$ as the eruption
progressed, as ${t}^{-0.74}$, whereas those of the broad components remained
essentially constant. Conversely, the line fluxes of the narrow components of
Pa,$\beta$ remained roughly constant, while those of the broad components
declined by a factor $\sim30$ over a period of $\lesssim25$~days. The behaviour
of the broad components is consistent with them arising in unencumbered
fast-flowing ejecta perpendicular to the binary plane, in material that was
ejected in a short $\sim3.3$-day burst. The narrow components arise in material
that encounters the accumulated circumstellar material. The outburst spectra
were rich in coronal lines. There were two coronal line phases, one that
originated in gas ionised by supersoft X-ray source, the other in shocked gas.
From the relative fluxes of silicon and sulphur coronal lines on day 23.4 -
when the emitting gas was shocked - we deduce that the temperature of the
coronal gas was $9.3\times10^5$~K, and that the abundances are approximately
solar.
