I have 2 questions
my parents actual visa on the passport expires in may 2013. If they come here in December will they be able to extend their visa for more than may?

2nd question:
when can they reapply the visa in nepal?
Is it after their visa is expired? Or can they it before 6 months of the expiration date?

Please share your knowledge

???

 Depends on case by case.
1. Visa may or may not be extended. If you have strong reason, it may get extended.
2. In nepal, I believe it after visa expiry

madtech,
here are the answers for the questions (to the best of my knowledge......)

I have 2 questions
my parents actual visa on the passport expires in may 2013. If they come here in December will they be able to extend their visa for more than may?
-- Visa doesn't govern the legal status of ur parents once they are inside the USA. I-94 @ port of entry is what you should be looking for.
Since ur parents visa expires in May 2013 (not sure of the exact day since u didn't provided one), legally speaking they can enter USA until the day, the visa expires, although that is not a good idea nor suggestions. Usually, officer @ port of entry for B1/B2 gives for 6 months of stay in I-94. Now in your case they might just give until the date of visa expiriration, which is May 2013. Once they are inside USA, they can file for extension of stay (basically u r not applying for visa rather extension of I-94). Basically this is done usually before or around 90 days of I-94  expiration date. Now the reasons can be anything for their extension, But I guess you know the valid reasons are ;). Legally u don't have to leave the country until decisons have been made to ur case. Just in case its rejected then leave ASAP. there is no waiting period nor any no of days. so lets say ur parents got i-94 until 30 may 2013. U filed for extension of another 6 months on march 30 2013.
Then they can stay in the country until u receive decison from USCIS.

2nd question:
when can they reapply the visa in nepal?
Is it after their visa is expired? Or can they it before 6 months of the expiration date?
-- Usually the best time is between 90 days to 180 days from the expiration date of visa. Doing after its expired is not a bad idea, but doing before 6 months is not a good idea as you have enough time on ur visa for a leasiure travel.

But in ur case, if they are travlling on dec 2012 and visa doesn't expire until april 2013, it is better for them to come here and apply for their extension for I-94.

Again, VISA is just needed to enter the country, I-94 is what they need to legally present in the United States.

Hope this helps

I have a hunch that if they apply for the extension in the US, that might count against them when they apply for visa later in Nepal.  I have no factual basis for this, just a hunch.