If I am to write a questionnaire to find out why students start on a e3, l1, l2 English course and also why they stay, what kinds of questions should I be asking bearing in mind their level of English.
I want to find out what motivated them to start and to persist.
This could be useful for you (especially appendix 1)
Motivational Orientation in English Language Learning - Articles - UsingEnglish.com
You could ask them whether they are overtaxed or underchallenged. With regards to this, why they decide to stay in the course.
Do they feel more confident to attend a course level which is below their skills?
Do they need challenges to grow their full potential and enhance their motivation?