Hybrid sub-assembly for driving a vehicle, having: - at least one primary shaft; - at least one secondary shaft; - a transmission gearbox comprising at least one intermediate shaft different from the primary shaft and the secondary shaft and sets of gear wheels (38, 28, 40, 30, 32, 42, 36, 48, 44, 46) for obtaining a plurality of gear ratios between the primary shaft and the secondary shaft via the intermediate shaft; - an electromotive unit comprising at least one reversible electric machine, and a coupling device which, in at least one intermediate coupling position, kinematically connects an output shaft of the reversible electric machine to the intermediate shaft, wherein the coupling device, in a secondary coupling position, kinematically connects the output shaft of the reversible electric machine to the secondary shaft without going via the intermediate shaft and, wherein the coupling device comprises at least one coupling mechanism (59, 159, 259, 359) permanently kinematically connected to the output shaft of the reversible electric machine, an intermediate speed reducer (260, 360) permanently kinematically connected to the intermediate shaft and a secondary speed reducer (264, 564) permanently kinematically connected to the secondary shaft, characterized in that the secondary speed reducer (264, 564) comprises a reversing gear wheel rotationally guided by a guide bearing that is coaxial with the intermediate shaft. - at least one primary shaft; - at least one secondary shaft; - a transmission gearbox comprising at least one intermediate shaft different from the primary shaft and the secondary shaft and sets of gear wheels (38, 28, 40, 30, 32, 42, 36, 48, 44, 46) for obtaining a plurality of gear ratios between the primary shaft and the secondary shaft via the intermediate shaft; - an electromotive unit comprising at least one reversible electric machine, and a coupling device which, in at least one intermediate coupling position, kinematically connects an output shaft of the reversible electric machine to the intermediate shaft, wherein the coupling device, in a secondary coupling position, kinematically connects the output shaft of the reversible electric machine to the secondary shaft without going via the intermediate shaft and, wherein the coupling device comprises at least one coupling mechanism (59, 159, 259, 359) permanently kinematically connected to the output shaft of the reversible electric machine, an intermediate speed reducer (260, 360) permanently kinematically connected to the intermediate shaft and a secondary speed reducer (264, 564) permanently kinematically connected to the secondary shaft, characterized in that the secondary speed reducer (264, 564) comprises a reversing gear wheel rotationally guided by a guide bearing that is coaxial with the intermediate shaft. wherein the coupling device, in a secondary coupling position, kinematically connects the output shaft of the reversible electric machine to the secondary shaft without going via the intermediate shaft and, wherein the coupling device comprises at least one coupling mechanism (59, 159, 259, 359) permanently kinematically connected to the output shaft of the reversible electric machine, an intermediate speed reducer (260, 360) permanently kinematically connected to the intermediate shaft and a secondary speed reducer (264, 564) permanently kinematically connected to the secondary shaft, characterized in that the secondary speed reducer (264, 564) comprises a reversing gear wheel rotationally guided by a guide bearing that is coaxial with the intermediate shaft.