Student or Learner
Do you use who before the verb and whom after the verb?
Subject: Who greased that vine? (Tarzan's famous last words)
Answer: He did.
Object: Whom did you see that night?
Answer: I saw him.
However: "Whom" is now seen as old-fashioned. You'll find most speakers now use "who" all the time. So for good, modern, idiomatic English, you don't need to worry about the difference at all.
It's alive and well after a preposition, but the trend seems to be to put the preposition at the end, except in formal usage. Very rarely do I hear someone say 'From whom did you buy it?', but I read 'to whom' and so on.
I think when the preposition is in the end is very easy for teaching English. The students easily give the answers and don't make mistakes.
Can the Dative whom substitute who to?
Who did you give the money?
"Who did you give the money?" doesn't sound right to me.
I think you need to be careful with labels. English doesn't have much of a case system left, and those old-fashioned text-books that talk about "dative" and "accusative" are, in my experience, making things a bit more complicated than they need to be.
"Whom" is not a "dative", it is an objective pronoun. "To who(m)" is also not a dative, it is the preposition "to" followed by an objective pronoun.
With verbs like "give", the indirect object is usually placed between the verb and the direct object -- and if it's a pronoun, the objective form is used. If the indirect object is put anywhere else, it is indicated with a preposition.
Sally gave Peter the book.
Sally gave the book to Peter.
To whom did Sally give the book? (Old-fashioned, very formal)
Who did Sally give the book to? (Modern, idiomatic)
Richard invited Mary to the party. (With "invite", you can't put the indirect object first)
Who(m) did Mary invite to the party?
As Russian has 6 cases, it's not at all difficult for us to determine whether whom is accusative or dative, Rewboss. We can't help using our native linguistic experience, which is much more often a hindrance, but sometimes it helps.
I don't insist all ESLs should differentiate between them, but I don't understand why you are so categorical (we all need to be careful with lables ).
You can't possibly deny it in I gave it to Mom and I gave it (a dog) a piece of bread reflects different relations between the subject and the object.