It's important to be polite when you decline an invitation. We normally give a reason why we can't do something and either apologize, or use words like actually or really.
在拒绝邀请时要表现得礼貌，这一点很重要。通常我们会给出一个不能去的理由，此外要么向对方表示道歉，要么用上像actually 或really这样的词。