>what should i use ?
>it is very tired for me to define different public variables in all forms
>because i duplicate forms and alter a little bit only.....

Avoid global variables! Use properties of your form. In stead of initializing the variable with

public gcVar1 && wrong way
gcVar1='abc'

you use

thisform.addproperty('Var1','abc')

Or you can use Form->new property in the designer.
To read and change the variable, you must add thisform. before the variable name

thisform.var1=10 && to change it
messagebox(thisform.var1) && to show it