General observation suggests that it is those who start to
learn English after their school years who are most likely to have
serious difficulties in acquiring intelligible pronunciation, with
the degree of difficulty increasing markedly with age. This
difficulty has nothing to do with intelligence or level of
education, or even with knowledge of English grammar and
vocabulary.
Of course, there is no simple answer to why pronunciation
is so difficult to learn — indeed, there is a whole range of
theoretical perspectives on the question. What is generally
accepted among psycholinguists and phonologists who
specialized in this area is that the difficulty in learning to
pronounce a foreign language is cognitive rather than physical,
and that it has something to do with the way “raw sound” is
categorized or conceptualized in using speech.
Many learners of English as a second language have major
difficulties with English pronunciation even after years of
learning the language. This often results in them facing
difficulties in areas such as finding employment. Up to a certain
proficiency standard, the fault which most severely impairs the
communication process in EFL/ESL learners is pronunciation,
not vocabulary or grammar.
A. Gilakjani and M. Ahmadi. Why is Pronunciation So Difficult to Learn? In: English Language Teaching, Vol. 4, No. 3. Richmond Hill: Canadian Center of Science and Education, 2011 (adapted).