What's the difference between "to you" and "for you".
For example: Here is a letter for you.
I'm writing a letter to you.

I've consulted the dictionary for the meaning of them.
It said: "to" , used to show the person or thing that receives something.
"for", used to show who is intended to have or use something or where something is intended to be put.
I, therefore, confused. How can I tell them? I often make the wrong sentences like "here is a letter to you". I think there must be some ways to tell them and use them like these kind of sentences. Pleas help me. Thank you.

Question

What's the difference between "to you" and "for you".
For example: Here is a letter for you. 
                    I'm writing a letter to you.

I've consulted the dictionary for the meaning of them. 
It said: "to" , used to show the person or thing that receives something. 
           "for", used to show who is intended to have or use something or where something is intended to be put.
I, therefore, confused. How can I tell them? I often make the wrong sentences like "here is a letter to you". I think there must be some ways to tell them and use them like these kind of sentences. Pleas help me. Thank you.

Answer

"To" is usually followed by a verb i.e. "I'm writing to tell you that xx..."
"For" is usually followed by a noun i.e. "Here's a letter for you".
This page contains more examples that might help: http://www.espressoenglish.net/difference-between-to-and-for/
However, there might be times when this isn't applicable. For instance, we say:
"I listen to music" 
"The session takes place from 5pm to 7pm" 
I'm not sure if there's a rule in this case, but as your listening skills improve, you might be able to get a sense of what sounds right.