This study aims to test the extent to which 120 Kuwaiti EFL learners are aware of the structure of relative clauses in English through measuring their ability to produce this structure. It also checks whether the English proficiency level of the participants plays a role in their answers on the test. For this purpose, a sentence combination task was used to measure the participants' ability to produce correct relative clauses in English. This test was selected in order to examine the types of error made by the participants pertinent to relative clauses in English. The results reveal that Kuwaiti EFL learners may not be fully aware of the formation rules of relative clauses in English (total mean = 60.4%). The t-test shows that the English proficiency level affected the participants' answers on the test. Specifically, there were statistically significant differences between the answers of the advanced (76.3%) and intermediate learners (44.4%). The number of errors made by the advanced learners was less than that made by the intermediate learners. Regarding the types of error made by the participants, the most noticeable ones were: deletion of the relative pronoun, wrong relative pronoun, repetitive use of resumptive pronouns, passivisation of the relative clause and problems with the indirect object and genitive relative clauses. The researcher analysed the participants' erroneous answers and provided potential reasons to account for them. Finally, the study concludes with some pedagogical implications and recommendations for further research.