This paper discusses Hong Kong Cantonese ESL learners’ judgment of the use of the English definite article. 33 Cantonese ESL learners majoring in English participated in the study, which consisted of a grammaticality judgment task requiring participants to judge the grammaticality of 60 sentences with regard to the use (or non-use) of the definite article. Two post-task interviews of four interviewees each were conducted to collect in-depth reports about learners’ thinking processes during grammaticality judgment and the strategies they employed. The results of the study show that the generic use of the definite article is largely unknown to Cantonese ESL learners, and learners make certain incorrect hypotheses about the use of the. Reliance on L1 translations is also observed, showing learners’ unawareness of the functional and semantic differences between similar items in the two languages. It is suggested that ESL teachers alert learners to their misconceptions and help learners eradicate their faulty assumptions. The generic use of the, alongside other structures used to represent generic reference, should be incorporated into the curriculum of an advanced ESL classroom. Future research about English article use should not only focus on analyses of learner errors but also tap into learners’ thinking processes.