這句句子的文法是否錯？

When had he been a teacher? 這句句子的文法是否錯？
No, the sentence is correct.

Q. When had he been a teacher?
A. He had been a teacher from 2001 to 2005.

Other possible questions:
Q. During which time had he been a teacher?
A. He had been a teacher from 2001 to 2005.

Q. How long had he been a teacher?
A. He had been a teacher for 10 years before he retired.

Likewise, "When have you been there?" is wrong but
"When had you been there?" is acceptable.

過去完成式又怎樣？

When had he been a teacher? 是否要改成 Since when had he been a teacher?

----When he been?
----When=time?
---------Error and Correction----------
(1)When had he been a teacher?-----use Since when?, What time? Which time?
(2)When has he been here=Answer:-He has been here for (-----).
----At what time has he been here?------use Until when? How long?-----

(1)Since when has he been a teacher?-----use What time?, Which time?,
(2)Until when has he been a teacher?-------use At what time?, How long?
----Or How long has he been a teacher?----use present perfect tense.

正確句子:

(1) Since when has he been a teacher?

(2) How long has he been a teacher?

2012-10-05 22:04:20 補充：
Answer:
(1) He has been a teacher since 1990.
(2) He has been a teacher for 20 years.

when he been a teacher.
when=時間'