Writing is said to be the most difficult skill in English, therefore most EFL students tend to commit errors in writing. In order to help the learners to successfully acquire writing skills, it is inevitable to analyse errors and understand their sources. This paper attempts to explore the significance of learners’ errors, the different aspects of interlanguage, the different approaches of errors, the sources of errors and the types of errors on the basis of some empirical data collected from high-school students. The focus of the data analysis is narrowed down to the use of different tenses, the structure of complex and compound sentences and the passive voice. Besides this the structures correctly used by the students will be given attention too. Finally, on the basis of the data analysed some implications will be drawn.