Question

split_values1 is an array and it contents some values, all i want to do is add those values to the textfield with a , separating them.

for(var i = 0; i < split_values1.length; i++) {
   $('#teach_subjects').value += split_values1[i]", ";
}

Answer 1

Use join on your array and val to set textfield value:

$('#teach_subjects').val(split_values1.join(', '));