The conundrum of the identity of that relative clauses has drawn a considerable amount of attention from researchers in various fields of linguistics such as historical linguistics, syntax, and semantics, to name a few. The central issues are i) the status of relative that, ii) fundamentally different syntactic analyses, partly due to the different view on the status of that, iii) alternation issue of that vs. wh-relative pronouns, iv) different semantic analyses of that relative clauses, v) discrete discourse function analyses of that relative clauses. Regarding these central issues, this study has three goals: i) to conduct a comprehensive literature review and examine these issues qualitatively, with the aid of corpora (ICE-GB and COHA used); ii) to provide some insight and resolutions, if possible, to each issue, iii) to expand the horizons of delineating the identity of that relative clauses more comprehensively than before. With the comprehensive literature review and corpus research, this study attempts to resolve and provide some solutions and suggestions to this conundrum. Finally, this study expands the horizons of delineating the identity of that relative clauses more comprehensively, so that it can be helpful in subsequent research.