I had the following questions on an exam:
SECTION 1: GENERAL ERRORS
The following sentences contain ONE grammatical or vocabulary error that has been underlined. Briefly explain why it is an error.
Question 1: Jessica boasted for having won the first prize.
Question 2: The teacher wondered where were the students at the time.
Question 3: The men are sociable beings by nature.
Question 4: He has not yet gone to the bed.
Question 5: His room's windows are open.
Question 6: It is everybody's duty to defend their country.
I understand the issues with questions 1-4, but why are the words/phrases in italics in questions 5 and 6 considered as an error?