<script>
function update(){
var sel = document.testForm.testSelect;
var val = sel.options[sel.selectedIndex].value;
var div = document.getElementById('testDiv');
if( val == 'da2' ){
var inp = document.createElement('input');
inp.setAttribute('id', 'newInput');
inp.setAttribute('type', 'text');
div.appenChild( inp );
}
else{
if( document.getElementById('newInput') ){
div.removeChild( document.getElementById('newInput') );
}
}
}
</script>
<form name="testForm">
<select name="testSelect" onChange="update();">
<option value="da"> Да </option>
<option value="da2"> Да 2</option>
<option value="da3"> Да 3</option>
</select>
<div id="testDiv"></div>
</form>