i have a WF in which fields are disabled and these fields are enabled once form is approved now how can i make fields mandatory once form is approved?

you should assign a validation rule to respective fields.

I'd assume you a field like 'Status' that tells you whether an item is approved or not. then your rule formula might look like:

Status == 'Approved' && IsNullOrEmpty(NamedControlToBeMandatory)

in the same WF, i have a field which has validation,the validation works fine when form is submitted but it shows error(invalid field) after form is again edited.Not able to figure out reason for it

post screenshot of whole error message, configuration of the rule and whole rule's formula