Generally, a verb of communication or motion goes with the preposition
to: You [write / read / carry / send / take / deliver] a letter
to someone. (write to, read to, carry to, take to, deliver to).
Generally, a verb of existence or possession goes with the preposition
for: [It is / I have / There's] a letter for someone.
_____________
for also means instead of, in place of, as a favor to. So,
I'll read it to you means I will read it aloud so you can hear what it says. I direct the sound
to your ears.
I'll read it for you means I will read it in place of you
(maybe because you forgot your glasses and can't read it yourself,
maybe because you don't want to be bothered, or any other
reason). This can be reading aloud or silently. It is done
as a favor to you.
I'm writing a letter to Jack means I'm writing a letter that I am going to send to Jack. Jack is the intended recipient.
I'm writing a letter for Jack means I'm writing a letter
in place of Jack. Jack would normally be writing this letter, but maybe
he broke his arm and can't do it. I write the letter as a favor
to Jack.
CJ