You were using a fraction as a variable name. The independent variable in the domain specifier is repeatedly assigned values and you can't assign values to a fraction (example: t/tf = 0).
I second Bill's comment that subscripts take work to do right. You can start by reading
about 60% of the way down, about the Symbolize function and the Notations Palette. A notebook Tech Support sent me about making subscripted variables is attached.
Attachment: SubscriptedVariables101.nb, URL: ,