We applied for the visa extension I-539 for the parents which is pending. The visa is expiring in few days. Do they need to leave before the visa expired ?

what happen if they overstayed and the I-539 get denied ? does it effects their next visit to USA ?

Please advise. Thank you in advance !!!

Your parents don’t have to leave the country if you apply for extension on time. It happen to my family member.

What if the visa extension get denied ? They will be out of status by then and won't come back to USA in future.

If their extension gets denied, as long as they leave soon, they won’t be considered out of status however their existing B2 visa will be cancelled as well because of extension denial. So they have to apply and go for the interview for new visa.

If their extension is denied then your parents visa will not be expired. They will mail a letter to leave the country as per USCIS given time frame but generally, USCIS grant the extension.

Keep in mind if they stayed more in US than in Nepal in a year, they will likely not get new visa once it expires. 6 months US 6 months Nepal fine. You don't want consular to think that you want to stay more in US than Nepal.