The subject of this thread was pre-discussed in another thread. But the original question in that thread was poorly worded, so I had to start a new thread.
My question has to do with a mysterious class of noun phrases formed by appending a prepositional post modifier to a main noun, like:
[main noun] [prepositional modifier]
an obligation to his friends
a commitment to excellence
a key to the front door
an answer to that question
an assistant to the manager
From these noun phrases complete sentences could be formed by inserting a proper form of "be" between the main noun and preposition modifier, with or without a slight modification of the main noun by replacing the indefinite particle with a definite particle, demonstrative, or possessive:
His obligation is to his friends.
His commitment is to excellence.
The key is to the front door.
This answer is to that question.
This assistant is to the manager.
For some reason, some of the above complete sentences are not good English according to some native speakers. Why is that?