On form frmPayroll, I want the CheckNr field on the Main form to group the subform checks and be able to add a new check.
I don't understand your schema. What are the Demo tables? Why is Marital status in tblpayrolltaxes? Should tblSalaryTable be called tblPayment? Why is salary info in tblEmployees?
tblEmployees should be demographic info.
tblEmpRate should be dated and have the hourly rate as of a start date. The end date can be null until the employee gets a raise at which point, you put in the end date.
Every payroll application has some number of deductions such as local, state, federal taxes, insurance, savings, etc. and other payments such as a shift premium or uniform allowance, etc So, you need another table that defines those rates.
tblCheck should have the employeeID, CheckNum, dateIssued, and maybe a status so you can void a check if you need to.
tblCheckDetails should have the calculated deductions, total salary, other payments. Then you calculate the net amount for the check by summing the individual items.
I suppose you need a way to cut special checks but I'm not sure what that would entail. You would have to charge tax so again you need child records but you probably wouldn't have any other deductions or additional payment types.
I think you need to go back to square one with your schema.
You are far better off getting quickbooks or something similar. QB does a good job of covering the basics. Most companies use a payroll service because they don't want to have to keep up with tax code changes which can be a nightmare.