Errors made by language learners in learning a language are regarded as failure of competence. Linguists believe that errors are committed when the learner makes use of the learning strategies. By analyzing the learner's errors, we can better understand his inter-language and his learning process. It's necessary to understand the roots of errors before they are eliminated. Students' misuse in vocabulary and grammar demonstrates that teachers should pay more attention to the reinforcement of knowledge delivered to the students. Only in this way will students' abilities be improved effectively.