Bluebook Rule 5.3(b)(i) Omissions, When using quoted language as a full sentence
If you quote an entire sentence, beginning to end, capitalization isn’t much of a problem. But when you begin a sentence with a quotation from another source and it isn’t the beginning of the sentence, you cannot arbitrarily change the capitalization in the quoted material without indicating the change. Do that by enclosing the capital letter in brackets [ ]. The brackets alert the reader to a change from the original.
For example: “[C]itation forms in The Bluebook are designed to provide the information necessary to lead the reader directly to the specific items cited.” The Bluebook: A Uniform System of Citation 2 (Columbia Law Review Ass’n et al. eds., 18th ed. 2005).