English is a grammatically complex language; ESL learners need grammar work at every level. Those just beginning to learn English must study verb tenses, the parts of speech and word order; more advanced speakers can begin to work on the different types and uses of subordinate clauses and modifying phrases. Trouble spots for ESL learners working on their grammar can include subject-verb agreement, pronoun cases and the use of the passive voice. Once a student's sentences are correct enough to be intelligible, he can begin to focus on the smaller details of the language, such as articles and the placement of modifiers.
ESL learners working on their vocabulary will find it helpful to study the roots of words. People who already speak Romance languages have a significant advantage here, because many of the words are related. For everyone else, knowing the parts of speech is handy. Instead of memorizing "stable," "stability," "stabilize" and "stabilizer" as four different words, for example, ESL learners can memorize the root of the word and then practice the suffixes used to change its part of speech.
Even once a student has a working command of the language, he may not be intelligible to native speakers if his pronunciation is poor. If his native language does not use all the same sounds as English, he may have to systematically practice making the unfamiliar sounds. Students learning the distinctions between the English sounds "r" and "l," for example, will likely have to physically drill those letters, practicing how to use their lips and tongues to create the sounds. ESL speakers do not necessarily have to eliminate their accents to be understood, but they do have to be able to make most of the sounds in the language.
If an ESL speaker's pronunciation is fairly accurate and her grammar is good yet she is still not being understood, syllabic stress might be the reason. When a non-native speaker stresses the wrong syllables in words, it might be difficult for others to understand her. Sometimes, ESL learners will drop syllables entirely. For example, in the word "casual," the two syllables in "-ual" are not separated by a consonant, and thus might not be stressed correctly. Native speakers often don't recognize a word when it is missing a syllable, even if all the remaining syllables are pronounced perfectly.