Final answer:
Use a block format for a citation when you have a long direct quote exceeding four lines of prose or three lines of poetry according to MLA guidelines, or 40 words for APA guidelines.
Step-by-step explanation:
In academic writing, a block format for a citation is used when you have a long direct quote. According to MLA style guidelines, a block quote should be used for quotations that are longer than four lines of prose or three lines of poetry. For APA style, the threshold is 40 words.
The quoted text is indented 1/2 inch from the left margin, and quotation marks are not used, making it clear that it's a separate element from the rest of the text. Following the block quote, you should include a parenthetical in-text citation. This format provides a visual indication of the extended quotation and allows you to present evidence or an argument in a clear and impactful way.