The objective of this research is to identify and explain the language errors in the syntactic field contained in the explanatory text and to describe the Indonesian teacher's efforts to improve the language skills of class XI students. This research uses descriptive qualitative method. Data collection techniques was in the form of observation, documentation, and interviews. Distribution method was used in the data analysis. The results of this study found that there are 11 sentences containing errors which was categorized based on phrase structure errors and sentence structure errors. Phrase structure errors were found in 3 types, in the form of 1 double plural error, 3 incorrect prepositional writing, and 3 excessive or redundant word use. In the sentence structure, there were 5 types of errors, which are 1 incorrect use of diction, 2 unnecessary interrogative sentences, 1 sentence without a subject and without a predicate, 1 sentence without a subject, and 1 illogical sentence. The teacher's effort in improving the language skills of class XI students was that the teacher shows errors in the results of explanatory texts made by students and has been given input about their mistakes, in this way students can learn language errors and avoid them in the next task of writing essays.