Is there any chance i could see your code, i am facing the same issues as well…  . My model seems to be repeating the previous word over and over again [ <SOS> , CAR, CAR, CAR, .... ]. The masking of tgt doesn’t seem to be doing anything at all.
Also, in terms of adding the SOS and EOS, how did you shift the sentence to the left for the teacher forcing component ?