1. foolish 既可以指行为、话语等“愚蠢的，傻的”，也可以指人“愚笨的，傻瓜似的”。如：

To accept their offer would be foolish. 接受他们的帮助是愚蠢的。

We must put an end to this foolish behavior. 我们必须终止这种愚蠢的行为。

He is a foolish young man. 他是个愚笨的小伙子。

You were foolish to believe what he said. 你相信他的话太傻了。

你的以下两句都是对的：

He is foolish.

It's foolish to do like this.

2. 下面句子中的现在分词短语 feeling foolish and watching him 为伴随状语，即伴随谓语动词 stood 同时发生：

I just stood there feeling foolish and watching him.  我只是傻傻地站在那里看着他。

3. It's foolish of sb. to do sth. 句式习惯上用 of，不用 for。又如：

It was foolish of me to forget. 我真蠢，竟然忘记了。

It’s foolish of you to believe her. 你相信她真是太蠢了。

比较同义句：

It was foolish of her to say such a thing.

= She was foolish to say such a thing.

她说出那样的话来，真是太蠢了。