In grammar, a subject complement follows a copular and describes the subject of a clause. Although nouns, pronouns and noun phrases most frequently perform the function, prepositional phrases can also function in the say way.
1) Can a prepositional phrase beginning (or ending) with 'to' follow a copular to indicate directions? If so, under what circumstances? For example, I don't believe it is grammatically sound to ask:
- Which train is to the city? (to ask for the train that will going in the direction of the city)
- Is this letter to John?
- Where are you to? (wouldn't be correct without 'off' before the 'to')
If they are indeed ungrammatical, why so? Why does 'to' strictly need a verb before it when 'from' doesn't despite their falling under the same category of words?
2) Whether the usage of prepositional phrases as subject complement is acceptable can greatly vary depending on the context. For example, it's perfectly grammatical to say:
- I am on all fours looking for my keys.
But what if the verb "BE" is behaving as a lexical verb, taking do-support in a present-tense negative construction?
- "Don't be on all fours, the ground is really dirty," said...
Is it still correct? If not, is there any way can I rephrase it without adding implications? Another example:
- "Don't be in this room!"
Many thanks in advance for answering.