Andrew,

Here's my explanation for pluralization. Because the table doesn't contain a car it contains cars. The row is a car the table is a set of cars. In your Sales Order example my naming would be:

SalesOrders - the table contains multiple sales orders not a single sales order

SalesOrderLines - Non-plural Sales Order because the each of the lines belongs to a single sales order, but the table contains many sales order lines

Naming conventions are always going to be debated, but to paraphrase Grant the important thing isn't what the standard itself is, but that you have and enforce one. In reality I'd name the tables sales_orders and sales_order_lines.