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 structure function $F_{2n}$ and spin structure function $g_{1n}$ through electron scattering on the (un)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.