If your green card expires or has already expired, you continue to be a permanent resident of the United States. You do not lose your immigration status. However, the physical card that proves your status is expired and generally not valid for most uses. And yet it’s not mandatory to renew an expired green card before applying for citizenship.
You still can apply for naturalization with Form N-400 with an expired green card. Although USCIS prefers that you always have a valid green card, this doesn’t mean your naturalization application will be rejected.
But a conditional green card is a different situation. If you obtained conditional residence through marriage to a U.S. citizen, the initial green card issued to you will last for only two years. The expiration of the green card is in fact the expiration date of the conditional status. Within the 90-day period before the card expires, the conditional resident should file Form I-751, Petition to Remove Conditions, with the spouse. Certain exceptions exist for petitioners that are no longer able to apply with a spouse due to death or divorce. Someone that has gained permanent resident status through marriage to a U.S. citizen can generally apply for naturalization after just three years. But first, the Form I-751 must be filed.
If you ask USCIS, they will likely tell you that an unexpired green card is required before applying for naturalization. In fact, carrying valid, unexpired proof of your permanent resident status is required by law. Besides there are plenty of reasons to keep your green card up-to-date and current (travelling, home loan, driver’s license, employment). Thus, if you are considering U.S. citizenship, file Form N-400 before your green card expires. It will prevent further possible complications.