>How can I change property (my added) of class from form of that class from some method?
>I have tried with command THIS but I didn't work (also I need explain of that command).

this refers to object itself. It would be a full reference to object that has property first :

this.myProperty && Directly object's property
thisform.myProperty && Form property
this.Parent.myProperty && Object's parent object's property
thisform.myContainer.myObject.myproperty

Cetin