区别是hard习惯用来指体力和精神方面所感受到的困难与艰苦，程度没有difficult深，但语气比difficult强，多用于口语。difficult习惯用来修饰需要运用智慧和技巧才能解决的问题，其程度要比hard深。tough指尖酸，艰苦，艰辛，多强调的是一种磨难。
一、hard
These apples are hard to reach. 这些苹果难够着。
It's hard to keep this secret. 很难保守这个秘密。
hard用作形容词时，还有“坚硬的”“严厉的”“冷酷无情的”之意，而difficult则没有.。
I like hard chairs. 我喜欢坐硬座椅。
Don”t be too hard on her. 别对她太严厉了。
hard还能用作副词，意为“努力地”，而difficult 则无此意。
Think hard and work hard. 认真思考，努力工作。
二、difficult
The question is too difficult for me to answer.。这个问题对我来说太难了，我答不出来。
We don't think it's difficult to learn a foreign language well ，我们认为学好一门外语并不难。
三、tough
What he said is so tough.他所说的真是刻薄!
It's really a tough task,tell me how did you do it?真是个艰巨的任务,告诉我你是怎么做到的?
A tough question,isn't it?尖锐的问题,不是吗?
A tough man.一个坚强的男人。