a letter for you与a letter to you

for是经过努力后给某人,write a letter for you.替你帮你写封,寄给给别人.
to是直接给某人,write a letter to you.给你写封信,寄给你.
但a letter for you要看语境,比如there is a letter for you.那儿有你的一封信.是寄给你的.