根据select选中的值去操作内容

<select name="searchfield" id="searchfield" onChange="getfield();">
    <option value="time" selected>时间</option>
    <option value="student">学员</option>
    <option value="teacher">讲师</option>
</select>
<input type="text" name="keyword" id="keyword" value="" placeholder="学习日期">

function getfield(){
    var s_field = $("#searchfield").find("option:selected").val();
    if(s_field == 'student') $('#keyword').attr('placeholder','学员电话/学员姓名');
    else if(s_field == 'teacher') $('#keyword').attr('placeholder','讲师姓名');
    else $('#keyword').attr('placeholder','学习日期');
}