1987 Nissan

best case scenario it's the clutch, worst case its the transmission can you start it and pop it in gear as you start it? also you can trying reving the engine up and poping it in gear should go in at the right rpm rev. so rev it to 4k or 4500 and trying pushing it into gear as the rpm go down. then you'll know if it's clutch or tranny 87 is about time for an upgrade. I love my old 83 but I replaced that 16 years ago, hope that helped