We have a Schengen visa valid for 6/19 to 7/19 for trip duration of 20 days. Can we enter say 7/10 and then leave before 7/30? Or does expiration date mean you have to leave Schengen area by then?
You have a Schengen visa with start/end dates of 6/19 and 7/19. Between these two dates you have been given a total of 20 days to spend in the zone. The issuer expects you to spend your 20 days between 6/19 and 7/19.
You are asking if it's OK to enter the zone on 7/10 and leave on 7/30 because your visa will be valid on your entry date and will have 20 days of unspent leave on it.
No. When a Schengen expires, all unspent leave is lost. In your case it means you will have 10 unspent days on 7/19 which are forfeit. Those days cannot be used beyond the expiry date.
Presumably the start/end dates and the 20 days sync up to the itinerary you provided when you obtained your visa. If there has been a material change in your itinerary, you can ask for your visa to be revoked and reissued to accommodate your new plans. They like it when people do this because it demonstrates a proactive attitude towards observing the rules.