Most of the time when you are writing an essay, you must use quotes to support your claim with evidence. When you do that, you write the sentence exactly as it is stated in the source even if there are misspellings or poor grammar. Then you place quotation marks around it to show that it is was not your writing. If you summarize or paraphrase, then you don’t put quotation marks around it because these sentences would have been written in your own words.
However, for both of these situations, you would need to cite and give credit to the author. Sometimes, not all of the sentence is needed to prove your point, so you may want to use the first part and the last part of the sentence is needed. An ellipsis is needed in the part left out.
If the original text contains bad grammar or spelling, you can correct it in your quote without indicating that you did so.
Quoting is, by definition, a word for word recreation of the original; however, you may need to add information for clarity, omit parts of long quotes, or indicate that a peculiar spelling or construction is accurate. Your style manual can show you how to make these changes to quotes correctly.