Dahon SUV D6 frame hinge
Oil it.
Your main hinge is a permanently assembled pin.  It is not a bolt and should not be removed.
I also notice your bike has rust marks - so I guess it has a hard life and is stored outside.  This will absolutely contribute to difficulties opening and closing the main hinge as well as shortening the overall life of your bike.
Are you following the recommended maintenance?  I expect it would say something about lubricate at least yearly, and more often in harsh conditions (like outside storage)