The subject of this thesis is the linguistic category of definiteness and indefiniteness, which is expressed in the English language via articles. The thesis consists of two parts: theoretical and research. In the theoretical part, the concept of definiteness and indefiniteness, the proper use of articles with nouns in a sentence, and the differences between definite and indefinite articles are explored. Different approaches to the category of definiteness/indefiniteness and some previous research on using articles in English as a foreign language is discussed as well. The research part of the thesis presents the results of a study conducted among first and third-year students of Čazma High School, specifically from the grammar and vocational programs of road traffic technicians. The research aims to determine how students master the use of definite and indefinite articles, and whether higher-grade students are more successful in mastering articles compared to first-grade students. It is assumed that higher grade students are more successful in using articles than lower grade students. The research results show that students in the grammar school program are more successful in using articles than students in the vocational and road traffic technician programs. The results also showed that students have generally mastered the indefinite article better than the definite article, and that third-grade students were more successful in mastering articles compared to first-grade students.