for and to

I sent the letter to you.

OR

I sent the letter for you.

"I sent the letter to you."
means:
I sent you the letter.
You are going to receive the letter.

" I sent the letter for you."
means:
I went to the post office to send the letter because you cannot send it yourself.
I am doing you a favor.

Some verbs behave this way with the prepositions "to" and "for".
The following are similar to the example above:

1. THROW
Throw the ball to him. (Hit him using the ball.)
Throw the ball for him. (Do him a favor. He cannot throw the ball himself.)

2. READ
The mother read a story to her child.
The translator read the document written in Japanese for Mr. Smith.

3. WRITE
Carlos wrote a letter to the president.
The lawyer actually wrote that letter for Carlos.

4. TEACH
Mrs. Chan taught Mandarin to the students.
When Mrs. Chan is absent, Mr. Liu taught (the students) for her.

5. EXPLAIN
Martin will explain the project to the Russian clients.
(Martin has not yet arrived.) Susan explained the project for Martin (instead of Martin).

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