A transmission may include a first input shaft, a second input shaft external and selectively connectable to the first input shaft, a third input shaft external to the second input shaft and selectively connectable to the first input shaft, an intermediate shaft, an idle shaft, and an output shaft, a planetary gear set mounted on the intermediate shaft and having first to third elements where the third element and one the first and second elements are selectively connectable to the intermediate shaft and the first and second elements are connected to the third and second input shafts, a plurality of gear sets allowing torque flows between the various shafts and the planetary gear set, first and second synchronizers on the output shaft, receiving torques from the input shafts, and selectively transmitting the received torque to the output shaft, and four clutches.