How do I include opportunity field variables in an email template?

When creating an email template in 7.6.2.1 Pro, I would like to include fields from an opportunity.  In the "Insert Variable" selection, I have the option for inserting Account, Contact/Lead/Prospect, and User module fields only.  I also tried both $opportunity_name and $opportunities_name as variables but neither is replaced in the email.  The "related to" object is an opportunity.  Thanks for helping.