Mispronunciation can sometimes be semantically fatal, i.e. mispronouncing a word may evoke another (I have heard "here" pronounced as "hair" in KSA). To approximate native pronunciation, lab sessions in the curriculum are advisable preferably taught by native speakers of English. If these sessions are unattainable, tutors can advise students to use their attentional processes and listening skills (to native speakers) to imitate native speaking. Nowadays, attention and imitation are back on the menu of cognitive linguistics. Language attitude may be responsible for lack of control of English pronunciation, i.e. if students' attitude about English is that it is the language of unbelievers, it is very likely that these students will be a failure.

Actually, I dont think making a mistake is so bad. I tell my students to not be afraid of making mistakes. The important focus should be speaking as much as possible. In time, some pronunciation issues can be resolved, by the usual method of making the student aware of the mispronunciation, giving them opportunities to hear the problem, and then giving them opportunities to start to feeling comfortable with correct pronunciation by describing the mouth positions, and having them practice either by repeating, singing, or recording themselves and then listening to themselves. Of course, teacher feedback is important at that point. Or if it a different problem other than correct sounds, for example word stress or sentence stress is the issue, they can listen to some of the many exercises now available on the internet and practice. However, controlled practice needs to be accompanied with speaking in meaningful situations.

What was said so far in a good analysis for the problem.

I coach actors and others on accents and language for theatre, film and television, and also work with non-actors on accent modification. Part of the problem could be a mapping of L1 phonology onto English, rather than learning how different the two languages are. If the emphasis in learning is on reading, students will frequently adopt “spelling pronunciations” rather than adopt the new sounds and sound rules of L2 (English).

All the answers above are right on the money. I would simply add, that a preliminary step is ascertaining that the user can hear the difference between the native and non-native pronunciations. This can be done using a same/different task at the phoneme, syllable and word levels (minimal pair). Once it has been determined that the listener can hear the differences, then articulation training of the desired phoneme/syllable can take place. This is not only an issue with L2 learning, but also with correction of misarticulation/mispronunciations in L1.

This video from vox does a great job of explaining why people struggle, using R/L issues in Asian languages as an example. https://www.youtube.com/embed/2yzMUs3badc

When teaching beginners, I start with what I call the "sounds of English". Then, we use the sounds as we decode words. For example, I teach that "s" has 2 sounds /s/ and /z/. We work on the sound. Then, when we are working with a word the student pronounces wrong, we decode or pull the sounds apart. I hear the individual sounds they make and we correct the pronunciation. Then, we put it back together again with the correct sounds. Having the student take the sounds of a word apart helps me in helping them to get the correct sounds for the correct pronunciation.

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I liked Sharon Hattaway's answer. I do review sounds with students. I include vowels and consonants, with a heavy focus on endings and word and sentence stress.

Thanks - I agree that the endings of words are VERY important. We not only do speaking exercises - we do listening exercises. They must learn to listen carefully and then reproduce what they hear. When you pronounce 70 "seventee" and you pronounce 17 "seventeen" the only difference is the"n", the last sound of the word.

I think the main cause is that some learners are unaware of the silent letters in some English words.

I am a brazillian and I had real trouble on learning English when I was younger. I think the real problem is that people in general compare their first language structure to English structure. For example: In portuguese, we can say either "I have a book" or "have a book" (the subject is perceptible on the verb desinence), but it doesn't exist in English. It's very confusing