为什么下面这句话中用了“it”而不是“him”？

为什么是are you he而不是are you him

本来人称代词作表语应该是主格代词

用宾格代词代替主格代词作表语是口语中的用法

him

it是形式宾语，指代to send products overseas.

一般情况下不定式作主语都要用it作形式主语来代替的。

Ⅰ.动名词和不定式做主语的相同点

1.不定式短语和动名词做主语时，常常为了保持句子的平衡，用it来做形式主语，如：

It is not easy to learn English well.

It is no use asking him for help.

2.如果主语用了不定式,表语和宾语也要用不定式;主语用了动名词,表语和宾语也要用动名词,这在英语中称为对称性,如:

To see is to believe. = Seeing is believing. (眼见为实)

(85)It is better to stay home than _A__ out.

A. to go B. going C. go D. gone

Ⅱ.动名词和不定式做主语的不同点

1.不定式做主语常表示具体的某一动作;表示泛指或一般的抽象的概念时,多用动名词,如:

(92)_B_ is a good form of exercise for both young and old.

A. The walk B. Walking C. To walk D. Walk

2.在”There/It +be+ no use(good/help/need)”之后常用动名词做主语,而少用不定式,如:

It /There is no use crying over the split milk.

It /There is no help taking this medicine.

★但是如果该句型中用了for引出不定式的逻辑主语,就只能用不定式,如:

(93) There is no good for us to do that exercise.

3.不定式可以和when, where, how, what, whether等连用,在句子中做主语,宾语和表语,而动名词则不能,如:

How to solve the problem is a hot potato.

Where to put the box seems hard to decide.

Whether to do it hasn’t been decided.

What to do next is up to you.

一、I don't know what to do 和 I don't know how to do it 中的 what to do 和 how to do it 都是不定式的复复合结构作 know 的宾语制。分别相当于下面的宾语从zd句：

I don't know what I should do.

I don't know how I can do it.

二、He teaches me how to study 不同于上面的结构。动词 teaches 后的 me 是宾语，不定式复合结构 how to study 是宾语 me 的补足语。