Okay...

To start, the boolean AND value is written as &&.

The if statement is one statement; both conditions must be grouped inside one set of parentheses, so it should be written like so:
if (Zahl > UGW && Zahl < OGW)

And then, you have to add ; after both return statements to identify the fact that that is the end of the line.

All together, your code should be:

1. boolean circa(float Zahl, float UGW, float OGW)
2. {
3.   if (Zahl > UGW && Zahl < OGW)
4.     return true;
5.   else
6.     return false;
7. }

Great, it works.

Actually, I favor an even shorter form:

1. boolean circa(float Zahl, float UGW, float OGW)
2. {
3.   return Zahl > UGW && Zahl < OGW;
4. }

Booleans are often underused...

Are you the same phi.lho from the autohotkey forum?

Yes... (Well, I am PhiLho in most Internet places, except when the site doesn't allow me to use this nickname. Otherwise I am Phi.Lho or Phi_Lho...)