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 ex- trapolation 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.  We 
comment on the influence of final-state interactions on these observables in kinematics at the EIC.