OnSubmit After Checking Return False And Still Submit

I am building a registration form and have some javascript that checks the forms user input. This is all activated when the submit button is pressed and the forms onSubmit event is called and runs a function.

I have tested me functions and even when it returns false the form still submits?

Im not sure how i can stop it submiting even if it returns false.

Then onSubmit called this function:

function subForm(theform){
if(checkForm(theform)==false){
alert("it is false");
return false;
}
alert("it is true");
return true;
}

checkForm is working and is throwing alerts when certain criteria is not correct in the form, and then i will get an alert "it is false" so i know that the script is running until then, but the form will still submit?