Efficient Neural Machine Translation with Prior Word Alignment

Prior word alignment has been shown indeed helpful for a better translation if such prior is good enough and can be acquired in a convenient way at the same time. Traditionally, word alignment can be learned through statistical machine translation (SMT) models. In this paper, we propose a novel method that infuses prior word alignment information into neural machine translation (NMT) to provide hints or guidelines for the target sentence at running time. To this end, previous works of similar approaches should build dictionaries for specific domains, or constraint the decoding process, or both. While being effective to some extent, these methods may greatly affect decoding speed and hurt translation flexibility and efficiency. Instead, this paper introduces an enhancement learning model, which can learn how to directly replace specific source words with their target counterparts according to prior alignment information. The proposed model is then inserted into a neural MT model and augments MT input with the additional target information from the learning model in an effective and more efficient way. Our novel method achieves BLEU improvements (up to 1.1) over a strong baseline model on English-Korean and English-Romanian translation tasks.