That question has many answers. The learner's first language will have a massive influence on this. I teach in Thailand and some of the things the Thais have difficulty with are the "l" sound at the end of a word. "Football" is said as "footbon" and Thai doesn't have the intial "V" so "Vodka" is "wodka". Contrast this with a German speaker. German has the "V" sound but not the "W". So they are polar opposites in this case.
You need to research the phonics of the students you are teaching.