A challenge for some ESL teachers is explaining how to pronounce the sounds of English properly. Some students can repeat new or unfamiliar sounds after simply hearing them pronounced by a teacher, but many cannot. Support the latter students with detailed directions on how to articulate these sounds. This requires that you understand the anatomical basis of the English phonetic system. Familiarize students with the speech organs (the different parts of the throat, mouth and tongue used in speech) and how they interact to form different sounds. Introduce students to basic speech anatomy with labeled illustrations, then demonstrate how they interact to form different sounds in English.
Many sounds in English have similar pronunciations and are difficult for some English learners to use correctly. For example, "p" and "b" are pronounced nearly identically, except that "b" is voiced (pronounced with the vocal cords vibrating) while "p" is voiceless (pronounced with silent vocal cords). Learners whose native languages lack one of these sounds often confuse them because they can't hear the difference. Unless given explicit training, speakers automatically filter out language sounds that don't occur in their native languages. Build student awareness of such phonetic contrasts with drills and games requiring students to identify or repeat the problematic sounds in similar pairs of words (such as "peat" and "beat").
It is helpful for ESL teachers to know not only the phonetic properties of English, but also those of the first languages spoken by their students. Languages vary in the number and inventory of sounds they use, and speakers of different languages will thus find different aspects of English pronunciation challenging. For instance, Japanese lacks the English "r" and "l" sounds, and Japanese learners of English often find these sounds difficult to distinguish and pronounce. Knowing students' language backgrounds helps you predict which aspects of English pronunciation will need extra attention in class.
The English spelling system is notoriously difficult to master because of its lack of consistent sound-spelling correspondences. This also makes it an impediment to English learners trying to determine the correct pronunciation of new words they encounter in writing. A useful strategy is to teach students to use English dictionaries and understand the phonetic transcription system used in dictionaries to represent pronunciation. This will not only build student awareness of the phonetic system of English, but also help them understand and visualize English syllable stress, another difficult aspect of pronunciation for some learners.