  Polarized electron-deuteron deep-inelastic scattering (DIS) with detection of
the spectator proton ("tagged DIS") enables measurements of neutron spin
structure with maximal control of nuclear effects. We calculate the
longitudinal spin asymmetries in polarized tagged DIS using methods of
light-front nuclear structure and study their dependence on the measured proton
momentum. Asymmetries can be formed with all three deuteron spin states ($\pm
1, 0$) or the two maximum-spin states only ($\pm 1$, involving tensor
polarization). The proton momentum dependence can be used to select pure S-wave
configurations in the deuteron and eliminate D-wave depolarization (transverse
momenta $p_{pT} \lesssim$ 100 MeV). Free neutron spin structure can be
extracted model-independently through pole extrapolation of the tagged
asymmetries. Such measurements could be performed at a future electron-ion
collider (EIC) with polarized deuteron beams and forward proton detectors.
