The English word that occurs in a variety of usages, the complexity of which tends to confuse EFL students. This study investigated what might be the hierarchy of difficulty among eleven usages of that. The usages were initially categorized into 11 different functions. To test the knowledge about these functions, 66 items were created with six questions per usage and given to 487 Korean high schools students. Mean scores calculated for each usage were taken as indicators of difficulty, and thus an initial hierarchy of difficulty with 11 levels was established. By means of repeated measures ANOVA, sets of means with no significant difference were merged into single categories of difficulty. This procedure resulted in four remaining categories in order from easiest to most difficult for students to learn. They were: (A) complementizer of a subject clause, demonstrative pronoun; (B) complementizer of an object clause, relative adverb, objective relative pronoun, complementizer of an appositive clause, subjective relative pronoun; (C) adverbial conjunction, demonstrative determiner; and (D) adverb. Cross-validation demonstrated that the same order of hierarchy was maintained for both schools and genders. An exception was found for a group with lower English proficiency where only the usages in category A were easier while those in the other categories were equally difficult. These findings may help EFL teachers and textbooks authors predict the relative difficulty that students might experience in processing and acquiring usages of that.