Okay.  Over four days trying to solve this and I officially have no idea what I am doing...

Background:
New to SF (About two weeks) (I Appologize in Advance)
Professional Edition (Think this is a big reason why I can't get this done with my experience level)
Created a Custom Object Called DRF which has a Master-Detail relationship with the Opportunity Object.  
The Opportunity Object has a Lookup relationship with the custom DRF object (Not sure if I need this)
Have watched and completed the 2016 Visualforce Workbook (A few times now)

What I want to accomplish:
I would like for my sales staff to be able to click a detail button from within the Opportunity Object that launches a visualforce page.  This page would include fields to input customer's responses.  (I want a visualforce page so that I can include helpful discovery questions for the users to have infront of them as well as links to more detailed discovery questions for them to utilize.)  These responses will reside in the DRF object.  

I will also use this data to populate e-mail templates for the users to send to clients to recap their discussions.  (We are in a heavy technical industry that can really get into the weeds quickly so it is nice to have a written copy of everything for them to review).

The problem I keep running into is I can not get the Visualforce page "pull" the DRF fields when I have the controller set to Opportunity. I had given up on that working so I went to using the DRF as the controll. Now I can figure out how to get the Visualforce page to update the correct record.

Page 13 of the workbook says that I should just have to {!opportunity.RDF__c.ExampleField} but I can never get that to work.  I have also tried __r and many other various ways.  

I really appreciate any and all help the community can provide.  I am hiring 4 new associates but really want to have this new system up before they onboard so we can get them up and running sooner!  Thanks again.

Ugly Code from a Newbee...
<apex:page StandardController="DRF__c">
  <!-- Page Header -->
  <apex:sectionHeader title="Discovery" subtitle="Discovery and Red Flag Report" />
  <!-- Begin Form -->
  <apex:form >
    <apex:pageBlock title="Discovery" mode="edit">
      <!-- How to Use Text -->
      <apex:outputPanel styleClass="rules">
        Use this Report to help build your Discovery Letter Email as the information you add will autopopulate into the email template.<br /><br />
      </apex:outputPanel>
      <!-- Fields -->
      <apex:pageBlockSection columns="1" showHeader="true" title="Prospect's Challenge">
        <apex:inputField value="{!DRF__c.The_Challenge__c}" required="false" />
      </apex:pageBlockSection>
       <apex:pageBlockSection columns="1" showHeader="true" title="Current Situation">
        <apex:inputField value="{!DRF__c.Current_Solution__c}" required="false" />
        <apex:inputField value="{!DRF__c.Electrical_System__c}" required="false" />
        <apex:inputField value="{!DRF__c.Other_Options__c}" required="false" />
        <apex:inputField value="{!DRF__c.Requirements__c}" required="false" />
      </apex:pageBlockSection>
            <apex:pageBlockSection columns="1" showHeader="true" title="Cost of Challenge">
        <apex:inputField value="{!DRF__c.Cost_of_Challenge__c}" required="false" />
      </apex:pageBlockSection>
            <apex:pageBlockSection columns="1" showHeader="true" title="Next Steps">
        <apex:inputField value="{!DRF__c.Next_Steps__c}" required="false" />
      </apex:pageBlockSection>   
      <!-- Button Section -->
      <apex:pageBlockButtons location="bottom">
        <apex:commandButton value="Save" action="{!save}" />
      </apex:pageBlockButtons>
    </apex:pageBlock>
  </apex:form>
  <!-- CSS -->
  <style>

    .rules {
      color: black;
      font-size: 14px;
      font-family: Arial;      
      margin: 10px 0 10px 30px;
      float: left;
      width: 100%;
    }
  </style>
</apex:page>