Hello.

What the difference between "Can you write a letter for me?" and "Can you write a letter to me?".
I do have a problem with "to" and "for".

Moreover, I mix up with sentences like "Oh my God! It's too hard for me" or "...It's too hard to me". And "English is an easy language for studying" or "to study".

As you can see, I'm absolutely zero.

Thanks in advance.