Problem: Produce a sentence that would be considered grammatically acceptable (OPTIONS:
- unacceptable
- acceptable)

Answer: I gave my brother a birthday present.

Problem: Produce a sentence that would be considered grammatically unacceptable (OPTIONS:
- unacceptable
- acceptable)

Answer: He has been must being interrogated by the police at that very moment.

Problem: Produce a sentence that would be considered grammatically acceptable (OPTIONS:
- unacceptable
- acceptable)

Answer:
The passengers who had known that the train was not on fire said that those who had thought so had barricaded themselves in the bathrooms.