Whether it is composing an e-mail, writing a small note in a TextEdit document, or entering text in a field in Safari, you might run into OS X using smart quotes, where the system will replace single or double quotation marks with directional ones that encompass a phrase or word. These can be useful for readability, but sometimes you might simply not want an auto-substitution, and instead prefer the straight quote marks you entered.
To avoid this, you have a couple of options.
Whenever OS X makes an automatic change to text formatting, either by correcting a word or substituting characters, you should be able to undo it immediately by pressing Command-Z. This is useful for if you mostly want smart quotes, and only a few times wish to use straight quotation marks.
Disable Smart Quotes
If you prefer to not have smart quotes entered automatically, then you can undo them either systemwide or on a per-application basis. For the systemwide settings, go to the Text tab of the Keyboard system preferences, where you will see a checkbox for using smart quotes and dashes. If a program continues to substitute quotes, then it may have its own settings for this behavior, so check its preferences for relevant options.
Right-clicking a quote selection will give you options in the Substitution sub-menu to manage it.
The problem with disabling smart quotes is if needed, then they will not be easily substituted. However, you can do this in most programs by highlighting the quote mark and then right-clicking it, where you can choose the “Replace Quotes” option from the Substitutions menu, or choose “Show Substitutions” to reveal a panel in which you can choose options for managing your current text selection.
The text substitutions panel, if selected, will give you options for setting and managing substitutions for selections.