Brackets allow you to insert your own words into quoted material. You can insert words in brackets to explain a confusing reference or to keep a sentence grammatical in your context.

The cost of treating obesity currently totals $117 billion per year—a price, according to the surgeon general, “second only to the cost of [treating] tobacco use” (Carmona, 2004).

To indicate an error in a quotation, insert [sic], italicized and in brackets, right after the error. If you use [sic] to indicate an error in your quoted source, be very certain that it truly is an error and that you know the correct form.