I'm not proficient in calculus. I'm having problem in understanding the statement in red. I think most of the time every function reduces to a constant once you insert the value of a variable. For example, f(t) = t^2 + 2t, will reduce to a constant value once you put a certain value of "t", say t=2, in the function. Isn't it so? What am I missing here? Please let me know. Thank you

RegardsIn a system of differential equations used to describe a time-dependent process, a forcing function is a function that appears in the equations and is only a function of time, not of any of the other variables. In effect,it is a constant for each value of t.

PG