I have something to do tomorrow?
Sorry but I am tied up all day tomorrow.
用I have something to do来表示您很忙，这也完全是中国式的说法。因为每时每刻我们都有事情要做，躺在那里睡大觉也是事情。所以您可以说我很忙，脱不开身：I’m tied up.还有其他的说法：I can’t make it at that time. I’d love to, but I can’t, I have to stay at home.