Error in A: The correct phrase should be "in trouble" instead of "at trouble."

• Correct sentence: We were in trouble because we had entered the building without permission.

Error in D: Since the sentence is negative ("hardly goes"), the tag question should be positive, so it should be "does he?" instead of "doesn't he?"

• Correct sentence: Your brother-in-law hardly goes to work by bus, does he?

Error in A: The sentence talks about an action that started in the past and continues until now, so the present perfect tense "hasn't worn" should be used instead of "didn't wear."

• Correct sentence: My brother hasn't worn that uniform since he left school.

Error in D: The sentence needs an additional conjunction or relative pronoun to connect the clauses properly, so "that" or "which" should be added before "must be important."

• Correct sentence: The letter that was sent by special delivery must be important.

Error in C: When using "either...or," the verb should agree with the subject closer to it. Since "her sister" is singular, the verb should be "does" instead of "do."

• Correct sentence: Either Lan or her sister does the housework every day.