Before we get started, let me just preface by saying I see this mistake a lot in manuscripts I edit, though the difference between the two words is quite easy to remember. I suspect that the problem may be some writers don’t realize that the word “foreword” exists.

“Foreword” is an introduction to a book, usually written by someone recognizable. To wit, “The famed astronomer wrote the foreword to the book about Mars.” The word is a combination of “fore” (which means “front”) and “word” (a synonym for “text”), indicating that the section readers are about the touch their eyes upon comes “in front of the book’s main text.”

All other meanings and uses of this word are spelled “forward” (which rough means “to move to the front”).