in my old translate system(Moses), unknown words are copied verbatim to the output.
in openNMT, when I translate a sentence, often give the wrong unk word in the results,especially if the first vocabulary is unk.
bitterness fed on the man who had made the world laugh .
the bitterness is unk word, will got wrong result:
fed on on . 的 人
the bitterness has been lost in result.
@guillaumekln give me explain, but I think It is very important for a practical system.