How can 那本书找了很长时间也没找到 be corrected? This sentence comes from my homework.
   那本书找了很长时间也没找到. 

(1) What's wrong with this sentence?
(2) How can this sentence be corrected?
A subject in the sentence is missing
The following sentences would all be correct:
那本书我找了很长时间也没找到.
那本书他找了很长时间也没找到.
那本书大卫找了很长时间也没找到.
We omit subject all the time in dialogues, but only when it is strongly implied the subject is known
Example:
A: &quot;我找到了不少作家X 未成名之前寫的書。&quot;
B: &quot;那作家X用不同筆名寫的那本你也找到了嗎？&quot;
A: &quot;那本书(我)找了很长时间也没找到。&quot;
In this dialogue, B knows the subject in the last sentence is A himself. therefore omitting 我 is acceptable on B's point of view. Nonetheless, 那本书找了很长时间也没找到 is still ungrammatical
Note: You can replace 很长时间 with 很久. The meaning will not change
The above answer is based on Standard Written Chinese (SWC)