Dave (with correction by Jay) has told you how to prevent this, but you may want simply to understand the conditions under which this will happen:
- A single hyphen between words will always remain a hyphen.
- A single hyphen preceded and followed by spaces will be converted to an en dash after you have typed a word following the space.
- A single hyphen preceding a word (without a space), such as "-ending," will be converted to an en dash when you finish the following word.
- Two hyphens preceded and followed by spaces will also be converted to an en dash, but two hyphens between words, without spaces (or two hyphens followed, but not preceded, by a space), will be converted to an em dash.
- Experiment will determine the results of other combinations of hyphens and spaces.
The rationale for the above conversions is that a spaced en dash (case 2) is used in the UK where we in the U.S. use an em dash (without spaces). These conversions make it easy for writers to insert these punctuation marks. What they do not facilitate is
inserting an en dash without spaces, as you might need between inclusive page numbers or a game score. For that, you need to use a keyboard shortcut (either the built-in one or a custom one that you define).
I had defined keyboard shortcuts for en and em dashes (and en and em spaces) before the AutoFormat As You Type feature debuted, and I still use them most of the time in Word. For some reason they work only intermittently in Outlook, so I'm grateful for the
AutoFormat As You Type option.