Pay someone vs Pay to someone

Question

I know we don't use "Pay to someone," but is there a reason? In terms of grammar, it seems correct.

Pay (transitive verb) + someone (direct object) Pay (intransitive verb) + to someone (prepositional phrase)

We say "Climb the mountain," and "Climb up the mountain" both.

Why don't we use "Pay someone," and "Pay to someone" both?

*I am asking this question because there may be something I don't know that may help improve my English.

Thank you in advance.

Answer 1

The verb "pay" can have two objects. Perhaps called direct object and indirect object in some descriptions. But you are not required to use both.

You can say "Pay Tom $5".
You can say "Pay Tom."
You can say "Pay $5."
You can say "Pay."
Do not say "Pay $5 Tom."

But use the "to" instead of the indirect object usually after the direct object:
You can say "Pay $5 to Tom."
Perhaps you could say "Pay to Tom $5," but that would be unusual.

I guess if the $5 is replaced by something long and complicated, then it is OK:
"Pay to Tom the amount you would have paid if he were not your brother-in-law."

Answer 2

Pay is a ditransitive verb like give and offer, so it takes two common arrangements of arguments.

John gave the book to Jane. Jane paid £5 to John

John gave Jane the book. Jane paid John £5.

But unlike give, we are not necessarily concerned with the direct object (the amount), and so we can omit it.

Jane paid John.

Show works the same way: even though what is being shown is important, we can omit it when it has already been mentioned:

Jane showed Helen [the picture].

But with give we cannot omit the direct object even if it has already been mentioned:

*John gave Jane.

*John gave to Jane.

are both ungrammatical. We have to say

John gave it to Jane.

Oddly, you can turn this round, but only if you use a pronoun for the recipient:

John gave her it.

but

?John gave Jane it.

is not acceptable to most English speakers.