Suppose F max=10N for a block . and suppose we apply 8N,there is an equal and opposite force of 8N applied by friction (according to Newtons third law).thus the block will not move .but if 20N is applied ,friction offers 10 N of opposite force and thus the block moves forward with 10N of net force .bUt Newtons third law is violated here when we apply 20N and friction applies 10N..why??

newton

in 1st case it is not NLM-3 which prevents the block from moving rather it is the friction. A/t NLm-3,  the  action and reaction pair forces are opposite in direction and their force of action is on different bodies. friction force and force apllied by you are not action and reaction of pair of each other and they are also acting on the same body. friction froce is due to the molecular interaction. when you apply 20N force on block then block also apply a 20N force on you, these are the action and reaction pair.