enough一词在英语中既可放在被修饰的词之前，又可放在被修饰的词之后。因此很多同学觉得enough 很难理解。其实这涉及到enough的词性问题。请看下面几个例句：
1. He is not old enough to go to school. 他年龄太小，不能去上学。
2. But there were not enough people to pick them. 但是没有足够的人采摘它们。
3. Six bottles of wine will be enough. 六瓶葡萄酒就够了。
要想搞清楚上面3个句子中enough的词性及含义，得先掌握它的用法规律。
[一号档案] enough作副词用时，意为“充分地；足够地”，通常要把它放在所修饰的形容词、副词或动词的后面，常与不定式或介词for连用。如：
1. You don't practise enough at the piano. 你钢琴弹奏练习得不够。
2. Is the river deep enough for swimming? 河水够深来游泳吗？
[二号档案] enough作形容词，意为“充足的；足够的”，在句中可作定语或表语。作定语时，一般放在它所修饰的名词前面。如：
1. I have enough money to buy a colour TV set. 我有足够的钱买台彩电。
2. Fifteen minutes is enough time for you to have a coffee. 15分钟够你喝杯咖啡了。
[三号档案] enough也可作代词用，意为“充分;足够”。如：
I have had enough, thank you. 谢谢你，我吃饱了。
看了以上有关enough的用法档案，同学们不难发现：在例句1中，enough是副词，作状语，用来修饰形容词old；而在句2中，enough是形容词，作定语，修饰名词people;句3中enough 是代词。
参考资料：前人回答