In Google AdWords, the plural version of a keyword is not the same as its singular version. As a result, Google counts plurals as separate from singulars in search query reports, when determining which ad to show for the search results, and when determining keyword match types. It is therefore incredibly important to include both the singular and plural versions of a keyword in your campaign, and to consider the ROI/search query results of the singular and plural versions of a keyword separately from each other. It may not make sense to run on “shoe” but it may make sense to run on “shoes,” or vice versa.
Caveat: While search query reports and the google algorithm will always consider the singular and plural form of a keyword as separate from one another, when using broad match or broad match modifier, Google will consider the advertiser to be advertising on both the plural and singular version of the word.
Caveat 2: The above is not true for MSN. MSN views singular and plural keywords as the same as one another, so if an advertiser is advertising for “shoe,” the ad will also show up when a user searches for “shoes.”