Language teaching requires good content. What principle would you follow to select and grade materials for language teaching?
When teaching language, one must be sure to include material the language learners are familiar with. What this means is: students of a new language are already nervous about learning a new language, therefore one should use materials which engage their interest by connecting with them on a personal level. By using materials which have been translated from their original language, or material which "speaks" about their heritage/culture, the material choice can put many at ease.
That said, one principle which could be very successful is the "Silent Way." According to Auburn University, there are five principles which can be used in language teaching: Grammar-Translation, Direct Method, Audio-Lingual Method, Silent Method, and Suggestopedia. (A link to the Auburn site has been provided below.)
As a teacher, I have found that students learn the most when placed in charge of their own education. The Silent Method allows the student to take charge of their learning, while the teacher/instructor acts as a facilitator. This engrosses the student in all aspects of the material, while the teacher acts as a guide.
In regards to grading, many learn from identifying their own mistakes. Therefore, by allowing the students to grade their own work, they can identify the problem and correct it as they come across each wrong answer.