Please look at this codes!!!

i cant understand the idea of how maple works with parameters...

if u watch it exactly u can see that first maple understands that diff(a__1,t) is equall 1; but anfter that it couldnt load the value 1.
why?

what is hapening?

what can i do for this problem. its not the main code. i just send u an example of what  i want and what maple does. plz help me. tnx