foolish指“无头脑的”、“缺乏常识的”、“缺乏判断能力的”.例如：
It was a foolish thing to pick a fight with his neighour.和他的邻居找碴儿打架是一件蠢事.
He is foolish to throw away such a chance.他真蠢,丢掉了这么一个机会.
It is foolish of you to say so (you are foolish to say so).你说这话真蠢.
Stupid指“智力差的”、“反应迟钝的”.例如：
Mary made a stupid mistake.玛丽犯了一个笨拙的错误.
He is very stupid in learning maths.他学数学很笨.
Henry is too stupid to understand such a complicated problem.
亨利太笨了,他不理解这么复杂的问题.
Silly指“头脑简单瓣、不懂事的”、“傻头傻脑的”.例如：
Don’t be silly!别傻!
Stop asking such silly questions!别再问这样傻的问题了!
It was silly of you (You were silly)to trust him.你真傻竟相信了他.
Silly有时带感情色彩.例如：
You silly child.你这个傻孩子.（并无太多的贬意）
概括起来,foolish相当于汉语的“愚蠢”,stupid相当于“笨”,silly相当于“傻”.
另外,我们也用fool这个词.Fool是个名词,意思是“傻子”（常指先天性的智力不全者）.例如：
Don’t act like a fool.别像一个傻瓜似的.