Can I write a letter threatening a lawsuit, if the suit has not been filed?

You can write such a letter. The question is whether it will help or hurt your case. It is always best to let the attorney who will represent you make such decisions to prevent introducing issues that will make your case more difficult to win.

Do you mean a settlement demand letter? Certainly contact an attorney. Don't venture into this without sound, professional expertise... especially when the other party is likely to seek representation upon receiving your letter.

No .

yeah if you can not afford lawyer. Than research the laws according to what state you live in. I think people need to really study and understand before they call the shots.