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 $g_{1n}$ 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.