Awareness from both teachers and learners that individual differences play a crucial role in the language learning process is essential to understand the success or failure of any student. This study intends to show the value of designing a learner profile to examine the written output made by an L2 Spanish speaker of English in different genres in order to discover her main areas of difficulty in the acquisition of the language. The method used consists of a systematisation of errors and a comparison of these results, which reflect her active knowledge, with her passive knowledge of the language by means of a questionnaire.