@Tim Edwards hello in my design 5v device and 1.8 device nfet psubstrate i wanted to connect through metal to ground but its getting shorted and the error behind i got to know is that nfet psubrate is getting connected instead of metal to ground how to overcome this could you please suggest me

The nFET substrate is connected to ground. What are you trying to do?

without the paint isosub it shows the error as analog and digital ground is shorted i was trying to extract spice file from magic when paint isosub to analog devices only there is no short when there is no paint isosub it shows that analog and digital ground as shorted

Yes. The substrate is global and everything connects through the substrate. One way to prevent device grounds from shorting through the substrate is to draw an isolated substrate region. The only way to actually isolate the device grounds is to put your analog devices in a deep nwell structure.

@Tim Edwards thank you so much for the information