下列三種說法是否都可以接受？

You must be Polish.
You must be the Polish.
You must be a Pole.

(1)is correct. "Polish" is used as an adjective here to mean "of Polish origin or nationality".
(2) is incorrect, because "Polish" as a noun can only mean the language of Poland.
It should be "You must be the Pole". The use of "the" specifies a particular Pole, for example, "... whom I have heard about so much."
(3) is correct. "a Pole" = "a person of Polish origin or nationality".

2012-07-04 11:44:59 補充：
"The poor" 是 "the + 名詞" 不是 "the + 形容詞"。
以下句子裡的"young, old, rich, poor" 都被用作名詞。
- The young and the old
- the rich and the poor

2012-07-04 11:53:13 補充：
"Chiness", "Italian", "Japanese" etc., as a noun can mean either the language or a person of that origin or nationality. However, "Polish" as a noun can only mean the language, and "Pole" can only mean a peron of Polish origin or nationlality.

You must be the Polish person who speaks no Polish.
You must be the Pole who carries a pole around.

2012-07-07 05:41:25 補充：
Polish can be an adjective of Poland, but it is not an adjective of 'Pole'.

the + 形容詞 可表示一類人，例如 the poor ，意思為窮人。
而 the Polish 的結構也是這樣，請問何錯之有？

2012-07-05 20:20:29 補充：
知識長：可否說直接一點？我知第一和第三句都有人說，所以多數是正確的，但第二句就不知道了。

2012-07-09 20:45:03 補充：
I see, thank you

1)
"You must be Polish" is CORRECT.

2)
"You must be the Polish" is INCORRECT.
There is no need to put "the" before Polish. Eg: "You must be Chinese" is correct, while "You must be THE Chinese" is obviously incorrect.

3)
"You must be a Pole" is GRAMATICALLY CORRECT, but it sounds a bit awkward, and most people will say "You must be Polish" instead.

For eg, we will usually say "You must be English" instead of "You must be an English man".

Thus,
1) You must be Polish
IS THE BEST OPTION.

All of them are actually OK, but you can improve by:
You must be a Polish;
You must be a Pole;
As for the second sentence, you can write it either like this:
You must be a Polish;or either
You must be the Polish I'm(or anybody) looking for.

You have to add an "a" in front of Polish/Pole. ie. You must be [a] Polish/Pole.
there is no need for a definite article "the".