Here are the corrections for each question:

Question 26: Should (A) need more (B) help, I (C) could call my (D) neighbors and my...
Correction: "Should" is correct, but "needed" should be "need" to match the conditional structure. The corrected sentence is: "Should (A) need more (B) help, I (C) could call my (D) neighbors and my..."

Question 27: Hardly (A) had he finished the work (B) than his wife (C) came in with two...
Correction: "Than" should be "when" to correctly follow "Hardly". The corrected sentence is: "Hardly (A) had he finished the work (B) when his wife (C) came in with two..."

Question 28: So (A) fluent (B) does she speak the language (C) that many people think she's...
Correction: This sentence is correct as it is. No changes needed.

Question 29: (A) No sooner when (B) had he got home (C) than he (D) was called back to the...
Correction: "When" should be "had" to correctly follow "No sooner". The corrected sentence is: "(A) No sooner (B) had he got home (C) than he (D) was called back to the..."

Question 30: (A) Under no (B) circumstances (C) you should (D) approach the man.
Correction: "You should" should be "should you" to correctly follow the inversion rule. The corrected sentence is: "(A) Under no (B) circumstances (C) should you (D) approach the man."