If you enter the United States on a K-1 or K-3 visa as the fiancé or spouse of a United States citizen, you can petition to obtain a green card. However, if you have been married for less than two years at the time you apply, you will be given a "conditional" green card.
Essentially, you have all the same rights and benefits with conditional permanent residency as lawful permanent residency, meaning that your time with a conditional green card can count towards the time required to qualify for naturalization. Conditional permanent resident cards can last for two years.
Following the issuance of the conditional green card, you can petition to have the conditions removed during the 90 days before it expires. If you are still married at the time of the petition, switching to a lawful permanent resident will usually not be hard to do. On the other hand, if your marriage has ended, it can be more complicated.
Petitioning for lawful permanent residency after your marriage has ended will require you to self-petition for permanent residency, which means you will have to prove one of three things:
- that your marriage was entered into in good faith and was terminated by divorce, legal separation or annulment;
- that you are the victim of spousal abuse or your child has suffered abuse from your spouse; or
- that leaving the United States would result in you suffering extreme hardship.
In cases where the couple is still married but no longer living together, the immigrant spouse will have to prove to U.S. Citizenship and Immigration Services that the marriage was real. This will require USCIS looking closely at the marriage to determine if it was bona fide.
Source: NY Daily News, "If you meet certain conditions, you can remove the 'conditional' from your conditional permanent resident card," Allan Wernick, April 18, 2012