Like everyone has posted, Grammar and deep knowledge of the laguage is the most important skill in language training. Good vocabulary boosts our self confidence as well. Over time, one should be able to guess what word will follow the given word or what sentence would follow the present sentence.. Another major aspect in language training is the correct pronunciation, for intsance, phonetics in English. One must take care to use simple,legible language instead of Shakespearan language ,because in the ned language is all about communicating and not about showing off our skills.
Knowledge about the language . Grammar . And capability of understanding any individual's capacity so as to enable effective teaching and learning , as a language trainer.