Whenever I type a single or double quotation mark, it turns yellow. Whatever I type next, an alternate form of that character appears. For example, if I type a double-quote then the letter A, I get á. What’s going on?
Stephen Mette

The Mac believes your single- and double-quote keys are operating like the Option key. Normally, when you hold down Option and type the letter E, an underline appears with an accent over it. If you then type a vowel, it will sport that accent – á, é, í, ó and ú, for example. Hold down Shift-Option and type the letter C, and you get Ç.

So something has changed the key assignments on your keyboard. In System Preferences, open Language & Text, then the Input Sources tab. Scan the list of keyboard layouts and you’ll probably see more than one item checked. The specific layout that causes this behaviour is US International – PC. Disable it and select British (assuming you’re in Great Britain), and your keyboard should now behave as expected.

Before you leave the Language & Text preference, enable the Show Input Menu In Menu Bar option. This places a flag in the menu bar that indicates the keyboard layout you’re using. Should this problem crop up again, a glance at the menu bar will tell you what’s gone wrong.

There’s one more thing you might do if you have multiple input sources selected. Glance up at the Input Source Shortcuts area of the Input Sources tab and examine the keyboard shortcuts used to select the next and previous input sources. If they’re key combinations that are easy to accidentally type (Command-Space, for example) you might consider changing them.

To do that, open the Keyboard system preference, click the Keyboard Shortcuts tab, select the Keyboard & Text Input entry from the left column, and choose the key combination from the right column. Now type one that’s more difficult to execute by mistake.