Actually it doesn't matter, but most of the cases, plural keywords will get more results. Google is smart enough to see singular and plural as the same.
Searching with plural, google will show also the singular keywords, that's why plural will get more results. While sometimes search "tutorial" will not show "tutorials". So you'd better use singular keywords in your title, description tags, because when people searching plural keywords, you also have the chance to show in SERPs.
For other search engines, I'm not sure.
|