A few pointers for you.
b/v, c, f/v, l/r, m/n, th... Then taking the Japanese alphabet into account, ka-ki-ku-ke-ko etc. most commonly you will hear each vowel sound at the end of English words i.e. "I amu goingu to youa hausu (house)." And many more.
Many people have been taught poorly, as in incorrectly (taking global English into account), and have been drilled with the translation theory. This means that the 'English mind' is very under-developed, with people's speaking fluency being slow to develop, as most people will have a dependency on the 'security blanket' of grammar rules. This is one big debate in Japan... But sure enough, you had better know your grammar (as a teacher should anyway) if you want to increase your success in the Japanese English education world.
Specific grammar problems include:
Perfect tenses. Clauses such as adverbial or adjectival. Articles a,an,the <-- big problem. Singular/plural - "I like dog." Or "I have two cat." Bear in mind these problems continue through intermediate to higher levels. The biggest obstical for a Japanese student is to understand the concept, especially when abstract. They are used to direct answers with straightforward rules, so you will also be educating them on using more than logic...
Many, many people either want to attain a high score with these English tests, or have to attain a certain score in order to attend university or gain employment etc. Cambridge tests such as the CAE or PET etc. are pretty meaningless in Japan. With tests in mind, I recommend learning about these to increase your knowledge of where many students are coming from & where they are going.
One more, gotta get back to work:
Students will often expect one answer. You will offer a choice, and they will want the best one. You will need to help them understand that language is personal, that we do not all speak exactly the same words. English is an artistic language, full of drama! You need to help them create their own 'dictionaries', where you give them three choices, and they should choose the one they like the most, or feel most comfortable with.
Japanese students in general can be very imaginative and very fun to work with, but only if you learn your trade. A poor teacher, or one that cares little for exploration and creativity, or simply with no professionalism at all (a high percentage of so-called 'teachers' in Japan that don't give two f***s for the job) will have a hard time educating their students efficiently and effectively.