|[ To whom ] did John reply t?||To whom moves as a unit.|
|[ Her emails ] were always replied to t at once ?||Her emails passivizes like a Direct Object.|
As these examples show, if a PP is obligatory, that is a pretty good sign the preposition is argument-marking.
Example Relation-name -ent. (a) John applied (for the job). apply_for yes (b) John relied *(on Mary). rely_on yes (c) John gave the book *(to Mary.) give (tradition!) yes (d) John sent the book (to Mary.) send yes
If these sentences have one relation each, it's got to be a different one in each sentence.
(a) John sat under the table. (b) John sat in the chair. (c) John sat in the hall. (d) John sat on the couch. (e) John sat on the table.
But to say that would be to miss that there is something common going on in each. Our clue is the existential entailment:
What is entailed is that there is something supporting John's seated position. Hence the basic relation seems to be sit_on: