When An introductory phrase begins with a preposition, a comma may derece be necessary even if the phrase contains more than three or four words. Use a comma to separate contrasting elements and if-then statements. Here’s a contrasting statement: I am writing a book, not an essay. Here’s an if-then https://www.saludcapital.gov.co/sitios/VigilanciaSaludPublica/Lists/Contactenos/DispForm.aspx?ID=2972663