The paper examines the use and understanding of English words and expressions in everyday communication of pupils in primary school in two different social environments. With the use of a questionnaire survey, compiled for the research, an analysis of use and understanding of the English words and expressions of Croatian-English sentences in learner’s interlanguage was conducted. The initial hypothesis is that there is a difference between self-assessment and real understanding of English words and expressions and Croatian-English sentences. The research was conducted on a sample of 83 primary school pupils in two different socio-demographic environments to determine the frequency of use of English words and English-Croatian expressions in everyday communication. Research has shown that pupils often use English words and expressions and that seventh-grade pupils understand them better. The findings of the study also indicate that there is no difference in the use and understanding of English words and expressions among pupils in a smaller settlement compared to the pupils in a big city.