The purpose of this thesis is to find out the importance of errors in students’ foreign language learning. In order to find out what kind of errors play a crucial role in the process of acquiring a foreign language, a study was carried out, in which fifty-eight students took part. A composition task, a multiple choice task sheet and a questionnaire was given to the students for completion so as to observe their different error categories, and to make comparisons between the groups at different levels. After data collection and analysis, the results of the study have shown that the influence of the mother tongue was greater in the case of the multiple choice test, however, in the composition task, developmental errors were dominant. From the results of the study, it can be concluded that students have to work on their language proficiency, and as far as their writings are concerned, they are very poor at grammar, so a new grammar teaching method should be introduced in order to be more successful in foreign language learning.