There exists another straightforward method, Erasure (Alvarez-Melis and Jaakkola, 2017; Arras et al., 2016;Zintgraf et al., 2017), which directly evaluates the word importance by measuring the translation performance degradation of each word.
it erases (i.e., Mask) one word from the input sentence each time and uses the BLEU score changes to denote the word importance (after normalization).