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 DIS cross section; (b) the structure functions of the process in the impulse approximation using deuteron light-front wave functions; (c) the extraction of free neutron structure by extracting the residue of observables at the free nucleon pole ("pole extrapolation"). As an application we consider the extraction of the free neutron structure function g1n through polarized electron scattering on the longitudinally polarized deuteron with proton tagging and on-shell extrapolation in the recoil momentum, possible at an EIC with polarized deuteron beams and forward detectors.
