How to insert variable from Opportunity into Email Template

I had posted this question elsewhere in the forum. Please pardon, if you are seeing it again.

I have created an Email Template like this.

Thank you for contacting us about $opportunity_name. We expect to respond to you by $opportunity_date_closed.

The variables that I inserted are from the Opportunities module. When I send the email, I go into a particular Opportunity, select Create Task, Compose Email and select the template under Option. I was hoping that when the email goes out, the Opportunity Name and Date Closed will be automatically filled in. But the resulting email appears exactly as shown above. Am I doing something wrong? Can somebody help me out here? Thanks a lot in advance.