Form I-751 is intended for use by conditional U.S. residents whose status was obtained through marriage to a U.S. citizen. Not everyone who married a U.S. citizen and then applies for a green card becomes a conditional resident; only those whose marriage was less than two years old when the immigrant was either approved for residence or entered the U.S. with an immigrant visa.
The status of these semi-newlyweds is made "conditional" because of U.S. government concern that such a new marriage might be fraudulent—that is, entered into for the purpose of evading immigration laws of the United States.
The two-year expiration date on conditional residence gives U.S. Citizenship and Immigration Services (USCIS) a second chance at seeing whether the marriage is in fact the real thing, and thus whether the immigrating spouse deserves a permanent U.S. green card (lawful permanent residence or LPR status). (See 8 U.S.C. § 1186a.)
There is some confusion about when one needs to submit the Form I-751 to USCIS.
If filing jointly with your spouse, you are supposed to submit it to USCIS within the 90 days prior to the two-year anniversary of having been approved for conditional residence. Some couples, however, mistake the second anniversary date as the one referring to their date of marriage. That date is irrelevant in this context.
Look on your conditional resident card, where you'll find your approval date. Start counting from that date forward two years, and then subtract 90 days to figure out the earliest date upon which you can file Form I-751.
Count the days carefully! Don't just think, "30 days is about a month, I'll subtract three months," or you might get it wrong. You want to make sure you start early enough to gather the necessary paperwork.
If you are not filing jointly with a spouse, however, the timing rules change. Conditional residents are allowed to file Form I-751 early or late if the U.S. spouse is deceased, the couple has divorced, the U.S. spouse treated the conditional resident spouse or child in a manner that was abusive or extremely cruel, or being removed from the U.S. would cause the immigrant extreme hardship. In such a case, you would also need to request a waiver of the joint filing requirement, and attach documents proving your reason to file solo.
If you file Form I-751 too early, USCIS will return your application to you and you'll have to refile it. This might take several weeks, which will delay your ultimate approval.
If you don't file your I-751 before your conditional resident status expires, your status will terminate automatically and you can be removed from the United States (deported).
If the failure to file on time is no fault of your own, you can, if you hurry, submit Form I-751 with an explanation and proof that your late filing is for valid reasons that were beyond your control. USCIS has the discretion to approve or deny your late filing.
Also, as mentioned above, late filing can be excused in cases where the applicant seeks a waiver of the joint filing requirement.
In any case, you'll want to get in touch with USCIS before your case is transferred to immigration court. Contact an immigration lawyer immediately if you're in this situation.
Filing Form I-751 is more complicated that you might think, requiring careful preparation and assembly of supporting documents. In order to have the best chances of getting it done on time and receiving USCIS approval, it is recommended that you get help from an experienced and competent immigration attorney. Hiring a lawyer will help ensure you avoid legal snags, so it's typically well worth the additional cost.