Answers for "when checkbox checked jquery"

jquery set checkbox checked

//jQuery 1.6+ use
$('.checkbox').prop('checked', true);
//jQuery 1.5.x and below use
$('.checkbox').attr('checked', true);

oncheck event jquery

$(".checkbox").change(function() {
    if(this.checked) {
        //Do stuff
    }
});

checkbox is checked jquery

$(your_checkbox).is(':checked');