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From my point of view is to listen and speak with the native english speakers.
Very important question indeed. The first and foremost requirement to teach English to non-native speakers is of course the best generic approah adopted which can be acceptable for the majority of students in general.
According to me, speaking only English.
In contemporary times there are a variety of ways by which the students' level of understanding can be evaluated, not just by teaching from course books. Modern technology, dynamic approach to learning such as Computer Assisted Learning Technique, word games and other Active Learning approaches adopted by almost all modern educational and language institutions are just a drop in the ocean to make the non-native speakers nearly perfect and fluent in Learning English as a second language.
Of course the main focus should be how the efforts from both the teacher and students can be mutually beneficial and fruitful.