How to make a values in a textbox not more than the other

You can use something like this as the custom Validation script of the second month field:

if (event.value) {
	var year1 = this.getField("Year1").valueAsString;
	var month1 = this.getField("Month1").valueAsString;

	if (year1=="" || month1=="") {
		app.alert("You must first enter the first period.");
		event.rc = false;
	} else {
		var year2 = this.getField("Year2").valueAsString;
		if (year2=="") {
			app.alert("You must first enter the years for this period.");
			event.rc = false;
		} else {
			var month2 = event.value;
			var duration1 = Number(year1) + Number(month1)/10;
			var duration2 = Number(year2) + Number(month2)/10;
			if (duration2>duration1) {
				app.alert("Error! The second period can not be longer than the first.")
				event.rc = false;
			}
		}
	}
}

With a few adjustments this code can also be used for the year field, although in that case you probably don't want to check that the months field is empty.

As the custom Validation of the fields in the first group enter this:

if (event.value) this.resetForm(["Year2", "Month2"]);

Adjust all field names in the code to match the actual ones in your file, of course.