In order to investigate the internal factors present in the interlanguage of learners of English as a foreign language, a qualitative research study was implemented to analyze the written productions of college students. The main objective of this case study was to identify the type of errors the students made and suggest the cognitive reasons that might have influenced them. The data were collected from a content exam applied to fourteen fourth-year students, and the syntactic errors were analyzed following the Error Analysis Approach. To classify errors is a complex task since the number of variables that influence the written production is considerable, and many times there is not a single cause of errors. However, the learners' errors do reflect the different learning strategies they apply, and it is possible to give an approximate description of this process, as it is shown in the results obtained in this study.