By default, the designer singularizes names of tables when reverse engineering tables to entity definitions when working database-first. This isn't always the most ideal choice: sometimes the tables in the database already have singular names or the table names are in a different language than English. To disable singularization, take the following steps. This is a global action and disables singularization of names in every element in the designer, until the steps are reversed.
Select Tools -> Bind Designer Events to Plug-ins from the main menu. In the dialog that pops up, select the Designer Event NamePluralToSingularConversion. An Auto-bound plug-in is bound to the event, called Plural to Singular converter plugin. Uncheck the checkbox under Is Enabled and click OK.
Singularization is now disabled, as there's no plug-in bound to the designer event. To enable it again, simply check the checkbox under Is Enabled for the plugin and click OK.