  We discuss the process of deep-inelastic electron scattering (DIS) on the
polarized deuteron with detection of a nucleon in the nuclear fragmentation
region ("spectator tagging"). We cover (a) the general structure of the
semi-inclusive DIS cross section on a spin-1 target; (b) the tagged structure
functions in the impulse approximation, where deuteron structure is described
by the $NN$ light-front wave function; (c) the extraction of free neutron
structure through on-shell extrapolation in the recoil proton momentum. As an
application we consider the extraction of the neutron spin structure function
$g_{1n}$ through polarized electron scattering on the longitudinally polarized
deuteron with proton tagging and on-shell extrapolation. Such measurements
would be possible at an Electron-Ion Collider (EIC) with polarized deuteron
beams and forward proton detectors.
