Saying no can be very difficult. People who are very shy often have little confidence.  They think that they have to meet everyone's need. It is very hard for them to say no. they sometimes feel shy if they say no, but doing things that they don't want to will make them unhappy.
A simple and direct “No” is the best way. If you want to explain, you can keep it simple. Just say, “No, I've already made plans for this time” or, “Sorry, I'm afraid I'm not free at that time.”
Other ways of saying no:
▲       I can't do this right now.
▲       No, but thank for asking.
▲       Please accept that I cannot come.
▲       I'd rather not.
To say no you should:
▲       Start the sentence with the word no.
▲       Shake your head when you say no to make sure they understand. Your voice should be clear and direct. Keep eye contact.
    If someone makes a request, it is necessary to ask for time to think it over. The decision should be up to you, but sometimes it is difficult to say no. You can try to reply in the following way:
   “Sorry. I would have taken care of your cat next weekend, but I will be going out. I know it will be hard to find someone to take care of the cat. I will try to help you next time.” Remember that: you have the right to say no!

2013年12月,上海市公安局经济犯罪侦查总队逮捕了一个骗税团伙。该团伙将普通的羊毛纱线谎报“羊绒纱线”高价出口,再以“棉纱线”的名义低价进口,就这样“空转”了4年,累计出口金额近70亿元,骗取出口退税款8亿余元。这说明（  ）

①公民要增强监督意识,积极关注国家对税收的征管和使用　②该团伙采取欺骗方法获得国家税收优惠,严重损害了国家利益　③税收的无偿性要求其具有固定性,绝不能随意改变税率　④税收具有强制性,纳税人必须依法纳税