try to do something like
<input type="text" name="input[<?= $i ?>]['subject']">
<input type="text" name="input[<?= $i ?>]['grade']">
<input type="text" name="input[<?= $i ?>]['year']">
try to do something like
<input type="text" name="input[<?= $i ?>]['subject']">
<input type="text" name="input[<?= $i ?>]['grade']">
<input type="text" name="input[<?= $i ?>]['year']">