同意曹老师的解答。

my knee 在通常情况下是不疼的，只有when I bend it 时，my knee 才疼。所以，it 指代整个when 从句的内容。it 不能单独指 my knee。

这种用法，在英语中是比较常见的。例如：

sb. likes it if (when) 格式：

在一些表示“喜好、厌恶”的动词之后，如enjoy, like, dislike, love, hate, prefer, appreciate 等，后接 if 从句或 when 从句，通常先在动词后接 it 作形式宾语。

She won’t like it if you arrive late. 她不喜欢你迟到。

He hates it when people use his bike. 他讨厌别人用他的自行车。

I hate it if you say such things in public. 我讨厌你在大庭广众之下说那样的事。

I’d prefer it if I didn’t have to do so much work. 要是我不必做那么多工作，那就太好了。

I would appreciate it very much if you would help me with it. 如果你能帮助我做这事，我会十分感激。

I don't love it when she tells me how to do things. 我不喜欢她对我做事指手划脚。

S + V + it + that 格式：

I take it you have been out. 我猜你刚出去回来。

You can put it that it was arranged before. 你可以说这是以前安排的。

I like it that you came. 你来了，我很高兴。

Rumor has it that the defence minister will soon resign. 据传闻，国防部长不久就要辞职。

S + V + prep + it + that 格式

说明：能用于此结构的动词不多，常见的有see to, look to, insist on, stick to, depend on, answer for 等。

I’ll see to it that all these letters will be sent to the post before twelve. 我将注意让所有的信件在12点以前送到邮局。

Look to it that this doesn’t happen again. 注意不要再发生这种事。

You may rely on it that he’ll come to meet you. 你放心，他会来接你的。

You may depend on it that he will turn up in time. 你可以相信他会及时赶到的。

I can’t answer for it that the boy is honest. (=I can’t answer for his honesty). 我不能保证这个 男孩是诚实的。

I can swear to it that this man stole our money. 我可以发誓，这个人偷了我们的钱。

再举一些例句：

I don’t like it when you shout at me.

I would appreciate it if you could let me know in advance whether or not you will be coming.

If I can get home by eight o’clock, I call it good luck.

If you could keep it to yourself I’d appreciate it.

He would much appreciate it if you could do him that favour.

She won’t like it if you arrive late.

it 除了充当主句的宾语，还可以充当主句的主语。

例如：

If you came earlier, it would be better.

It would be awful if he found out the truth.

It would be a shame if you lost it.

hurt在此意为“cause physical pain or injury". It 为人称代词，指代 when 从句的内容，即 I bend my knee. 此为人称代词的前照应用法，即 it 指代的内容在后面出现。

是的。