How do I stop Word from formating paragraphs not selected

[Guest]

When I highlight a section of text to underline or center after I select the
format button the entire document changes. When I hit undo just the
highlighted part changes. Am I doing something wrong or is the program buggy?

 

[Jay Freedman]

When I highlight a section of text to underline or center after I
select the format button the entire document changes. When I hit
undo just the highlighted part changes. Am I doing something wrong
or is the program buggy?

It's an obscure option that you need to reset. See
http://word.mvps.org/FAQs/Formatting/WholeDocumentReformatted.htm.