About Word Stemming
Google uses word stemming. Word stemming allows all forms of the word _ singular, plural, verb form as well as similar words and synonyms to be returned for a given search query. This can work both for and against a site depending on which form of a word a page is primarily optimized for. So if someone types in "house plans", not only will pages that are optimized for that phrase be returned, but so will pages that
contain all variations of that phrase,
for example:
house plan
house planning
house planner
Conversely, a page that optimized for _house plans_ will also be returned whenever a searcher types in any variation of that phrase. Using the same example, typing in any of the phrases below would also return a page optimized for _house plans_:
house plan
house planning
house planner
Word stemming is a helpful feature for searchers, since it saves them from having to think of many variations of a word. Word stemming can help as well as hurt your ranking for a given page as not only does it increases the number of words that you can rank well for (even if you do not include a given form of the word anywhere on a page) but it can also increase the amount of sites (competition) returned for a given search query. When you enter a search query in google, put a plus _+_ sign in front of the word for which you want to disable stemming for. For example:
house +plans
Would disable stemming on _plans_ and thus only return pages for _house plans_ and pages that contain variations on the _plan_ word.
Pay attention to stemming for your keywords _ particularly to what the _root_ word is and what Google considers to be a match for that word when optimizing pages over time.