It is not a simple matter of just writing a letter.
You should retain an experienced immigration lawyer to review all the facts and advise you accordingly.
J Charles Ferrari
Eng & Nishimura
The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship.
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When you are dealing with an I-601 waiver (or similar) and need to write a hardship letter, it is important to remember that it is not actually a "letter." It is actually supposed to be a whole story that is tailored to bring out certain characteristics of your relationship. It needs to be very well crafted in order to be effective.
You can find some information here: www.swagatusa.com/educational-materials but I strongly advise that you do not go at this without any assistance.