Many apps offer the option to turn off auto-substitution on quotation marks (") such as TextEdit, but many other apps do not have such settings, for example LibreOffice.
So how to type a normal quotation mark in those cases?
In OpenOffice and LibreOffice, you go to Format > AutoCorrect > AutoCorrect Options > Localized Options and uncheck the boxes for Replace under Single Quotes and Double Quotes.
Hold down the control key. It works everywhere that uses the System smart quotes. I don't know if LibreOffice does or not.