this代表單數概念，this是第三人稱單數主語。

主語是第三人稱單數時，be動詞用is

1. 如果在問句中用 this 或者 that 作主語，回答時通常用 it （而不用 this 或者 that ）作主語，即使指人也不用 he 或者 she 作主語；如果在問句中用 these 或者 those 作主語，回答時通常用 they （而不用 these 或者 those ）作主語。

例如：

(1) —Are those your sister's shoes?

—No,they aren't . I buy them for my mother.

(2) —The photo is very nice. Is this your mother?

—Yes, it is.

(3)—Is the woman your mother?

—Yes, . she is .

2.如果表語是單數名詞，主語可以用this 或者that，不能用 these 或者 those；如果表語是複數名詞，主語可以用 these 或者 those，不能用 this或者that。

例如：

This is my father.

That is my mother.

These are my parents.

Those are my grandparents.

These are doctors.

Those are teachers.

This is my pen.

That is your book.

These are my pens.

Those are your books.

this 和that一般用作指示代詞。但this和that還可用作副詞，表示程度，意思是“這麼，這樣地”、“那麼，那樣地”，一般用在非正式語體中，特別是在日常口語中。例如：

I didn＇t think we＇d get this far. 我沒想到 我們會走這麼遠。

The table is about this high and this wide. 這張桌子大約有這麼高這麼寬。

I didn't realize that things had got this bad.我沒有料到情況已經變得這麼糟糕。

I like him but not that much. 我喜歡他，但 還不至於那麼喜歡。

I would have walked out-I was that angry. 我本來是要走出去的-我是那麼地生氣。

I'm sorry, I didn't realize the situation was that bad.對不起，我沒有意識到情況是那麼糟糕

這個問題應該是指以this作主語的疑問句的回答為什麼不能用you are吧。

如：Is this your bike? 這是你的自行車嗎？這句疑問句的回答應該是Yes, it is. 或者No, it isn't.

不能使用you are是因為這句疑問句的主語是this，應該使用it來指代，而不是you。