The sentences that contain an example of contested usage are:
- The postman needs to know who this letter is addressed to so it can be delivered.
- And William also said that he did not understand why he couldn't play on the football team anymore.
The first sentence has contested usage around the phrase "who this letter is addressed to," which could be critiqued for ending a preposition with "to." The second sentence contains the contested usage of "couldn't" in the context of understanding eligibility for participating in a team, though it may not be as commonly debated as the first.
(Note: If the definition of "contested usage" leans more to grammatical debate, then only the first example might qualify distinctly. However, the second could still be included depending on context.)