A new diagnostic system has been developed for an interactive template-structured intelligent language tutoring system (ILTS) for Japanese-English translation where an efficient heaviest common sequence (HCS) matching algorithm and a ‘part-of-speech tagged (POST) parser’ play a key role. This is implemented by exploiting the system template which consists of a complex transition networks comprising both model (correct) translations and many typical erroneous translations characteristic of nonnative beginners all collected and extracted from translations of about 200 monitors. By selecting, from among many candidates’ paths in the system template, a path having a HCS with the student’s input translation as a best matched sentence, the template structure of the diagnostic system allows the potentially complicated bug finding processes in natural language to be implemented by a much simpler and efficient HCS string matching algorithm [20]. To improve the precision of a parser, we have developed a ‘probabilistic POST parser’ where we have eliminated ambiguity in part-of-speeches by manually pre-assigning POS tags to all words in potentially correct paths of the template. Combining the templatebased diagnostic system and the parser, we found that the ILTS is capable of providing most adequate diagnostic messages and a tutoring strategy with appropriate comments after analyzing the keyed-in translated sentences from students.