n order to specify the scope, you preceed your variable with the scope
type.  Therefore for a request-scope variable, the variable would be :
"requestScope.variable".  There are several scope types:

  pageScope
  requestScope
  sessionScope
  applicationScope

Additionally, when you have multiple c:if tags that test different
conditions of the same variable, I would suggest rewriting the block as
follows:

<c:choose>
  <c:when test="${requestScope.mode=='a'}">
    first thing
  </c:when>
  <c:when test="${requestScope.mode=='b'}">
    second thing
  </c:when>
  <c:otherwise>
    a catch all section
  </c:otherwise>
</c:choose>

Consider the above more like a switch or if/elseif combo rather than
multiple if statements.