- should be highly motivated to learn effectively (exams, job opportunities, out of interest, travelling, making friends)
- should get as much exposure to English as possible (podcasts, books, TV shows, living in an English speaking country)
- should learn English in English (and not be taught by teachers who teach using their mother tongue)
- should get regular feedback on their language errors (both immediate and delayed feedback)
- should do enough meaningful practice (especially speaking, it's not enough to just do grammar exercises in a book)
- should consider getting help from professional ESL/TELF teachers. If they do, they should go for those that have the necessary experience and qualifications.