Have you taken English for years, but people still don't understand you? I found this article on Wikipedia which outlines the problem pretty well:
Many teachers of English as a second language neglect to teach speech/pronunciation. Many adult and near-adult learners of second languages have unintelligible speech patterns that may interfere with their education, profession, and social interactions. Pronunciation in a second or foreign language involves more than the correct articulation of individual sounds. It involves producing a wide range of complex and subtle distinctions which relate sound to meaning at several levels.
Teaching of speech/pronunciation is neglected in part because of the following myths:
Pronunciation isn't important: "This is patently false from any perspective." Speech/Pronunciation forms the vehicle for transmitting the speaker's meaning. If the listener does not understand the message, no communication takes place, and although there are other factors involved, one of the most important is the intelligibility of the speaker's pronunciation.
Students will pick it up on their own: "Some will learn to pronounce the second language intelligibly; many will not."
Inadequate instruction in speech/pronunciation can result in a complete breakdown in communication. The proliferation of commercial "accent reduction" services is seen as a sign that many ESL teachers are not meeting their students' needs for speech/pronunciation instruction.
The goals of speech/pronunciation instruction should include: to help the learner speak in a way that is easy to understand and does not distract the listener, to increase the self-confidence of the learner, and to develop the skills to self-monitor and adapt one's own speech.
Even when the listener does understand the speaker, the presence of an accent that is difficult to understand can cause him to end the conversation earlier or avoid difficult topics.