These are parentheses: ( ) Don’t use them when brackets are called for.
These are brackets: [ ] Use them to enclose information in a quotation that is not part of the original. For instance, “A Ken Burns documentary shows him [Mark Twain] to have had several financial setbacks, primarily of his own doing.”
Brackets are also used following an error made by another, showing that you didn’t make the mistake yourself: “Trump once said that an accusation against him was unpresidented [sic].”
Sic is Latin for thus or so. Put it immediately after the offending word.