how can i write only true or false in textbox?

Yes you can but you probably need to enforce it? :-) Don't do it! This is hard to do, nearly impossible; more importantly, it will confuse your user beyond all expectations.

At the same time, the real solution is the right UI design. Use the following alternatives:

• Use two radio buttons is the same radio group: one "False", another one "True"; they will be checked on mutually-exclusive basis;
• Use a combo box with two list items: one "False", another one "True";
• Same as before, but use a list box.

—SA