做不到就不要轻易允诺，答应做了就要尽全力去完成。人无信则不立，做人不能出尔反尔，不要辜负别人对自己的信任，也不要挥霍自己对别人的信任，要时刻把自己的品德修养放在第一位，因为它是做人的根本。
成语出尔反尔用来比喻言行自相矛盾，反复无常，说话不算数，换句话说它包含违背诺言或食言的意思，那么可以用 break one's word，break a promise 或 fail to keep one's word/promise 等等，例如：
1. break one's word:
Tom said he'd help us move, but he broke his word and failed to show.
汤姆说他会帮我们搬家，但他食言没有出现。
If you keep flaking out, you're going to become known as someone who breaks their word.
如果你一直怪里怪气的，你会成为一个食言的人。
2. break a promise:
You promised not to gamble, but you broke your promise and went to a casino.
你答应不赌博，但你违背诺言去了赌场。
3. fail to keep one's word/promise:
The government failed to keep its promise of lower taxes.
政府未能兑现减税的承诺。
我们还可以使用一个短语动词来表示违背诺言，它就是 go back on，意为“食言，失约”等，例如：
He never goes back on his word.People are beginning to worry that the president is going to go back on his campaign promises.
除了表示违约或食言，短语动词 go back on 还有改变主意的意思，例如：The government looks likely to go back on its decision to close the mines.
然后无论上面中的哪个都很难体现出一个人反复无常的特性，于是乎诞生了一个很形象的习语 blow hot and cold，字面意思是“忽冷忽热”，那可不就是反复无常的意思吗？也可以用作 run hot and cold，常跟动词 keep 连用，相当于 chop and change，例如：
The boss has been blowing hot and cold about whether or not we're going through with this project. I wish she would just make a decision.老板对我们是否要完成这个项目一直反复无常。我希望她能做个决定。Her boyfriend keeps running hot and cold whenever she asks him to marry her.He keeps blowing hot and cold on the question of moving to the country.He's blowing hot and cold on whether or not to take the job.经过第一年的反复变换，他们已经定下了一个稳定的管理体制。After six months of chopping and changing, we've decided to go back to our old system.
当形容人反复无常时，还可以用 inconsistent 和 flighty，其中尤用于指女的，例如：
You are inconsistent and unpredictable.
你这人反复无常，真叫人难以捉摸。
Isabelle was a frivolous little fool, vain and flighty.
伊莎贝尔是个轻佻、没头脑的小女人，非常虚荣又反复无常。
词或词组辨析：
flake out 意为“行为古怪”
casino 意为“赌场”
