Semileptonic decays B! D−`+X were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A topological search for semileptonic B decays to resonant and non-resonant D−+ states was performed and the ratio of the branching fractions: Br(B ! D−`+X) Br(B ! D−`+X) + Br(B0 ! D−`+) = 0.19 +/- 0.10(stat) +/- 0.06(syst) was determined. Taking into account this contribution, the differential decay width of B0 ! D−`+ was measured as a function of the momentum transfer from the B to the D− in two separate analyses, using exclusive and inclusive methods of D− reconstruction. The distributions were fitted over the full momentum transfer range to extract the product of jVcbj times the normalization of the decay form factor F(q2 max):F(q2max)jVcbj = (35.4 +/- 1.9(stat) +/- 2.4(syst)).10−3. The value of jVcbj was computed using theoretical calculations of F(q2max), giving:jVcbj = (38.9 +/- 2.0(stat) +/- 2.6(syst) +/- 1.7(theory)).10−3. The total branching fraction Br(B0 ! D−`+) was determined to be:Br(B0 ! D−+) = (5:52 +/- 0.17(stat) +/- 0.68(syst))%.