So much wrong there...

a = 2;
tx = 2^(3/2);
x[t_] := a Cos[2 Pi t/tx] - Cos[2 Pi t];
y[t_] := a Sin[2 Pi t/tx] - Sin[2 Pi t];
f[t_] := x[t] + I y[t];

f[20]

(* -1 + 2 Cos[10 Sqrt[2] π] + 2 I Sin[10 Sqrt[2] π] *)


Don't use uppercase initials for your symbols - you might clash with built-ins.

Don't use some external user-defined global symbol inside a definition in the manner you have unless there's a good reason to do so (I left that as is, but it's bad practice).

Read the documentation, learn the difference between Set (=) and SetDelayed (:=).

Read What are the most common pitfalls awaiting new users? on this very site. Great resource for new users.

Go through the tutorials in the Mathematica documentation. Very worthwhile.

Recursion depth of 1024 exceeded during evaluation
I developed an Android application and I want to make a survey/questionaire about the usability, user interface, user experience etc. of my application to get some feedback from users. But I don't know which kind of questions to ask...

I would like to know If there are some links/tools/books that can help me.
