Clear text field when checkbox is unchecked

Couple of notes.

- To execute more than one line of code after an if-statement you have to use curly brackets around the whole code block.

- The default value of a check-box that isn't ticked is "Off", not "False".

- You can reset multiple fields in a single command by putting then in an array as the parameter of the resetForm method.

- I believe you defined your conditions the other way around. The "!=" operator means NOT EQUALS. You probably want to use the EQUALS operator, which is "==".

So combining it all your code should be something like this:

if (this.getField("Check Box12").value=="Off") {
    this.resetForm(["Applicant's Name", "pertyAddress", "Contact Name1", "Phone1", "Postal Code1", "Cell Phone1", "Email1"]);
} else {
    this.getField("Applicant's Name").value = this.getField("Property Owners Name").value;
    this.getField("pertyAddress").value = this.getField("Address").value;
    this.getField("Contact Name1").value = this.getField("Contact Name").value;
    this.getField("Phone1").value = this.getField("Phone").value;
    this.getField("Postal Code1").value = this.getField("Postal Code").value;
    this.getField("Cell Phone1").value = this.getField("Cell Phone").value;
    this.getField("Email1").value = this.getField("Email").value;
}