How to say "the one I sent the letter to"

Question

I sent him the letter - 彼に手紙を送ってあげた　

He was the one who sent me the letter - 手紙を送ってくれたのは、彼だった

He was the one I sent the letter to - ???

Answer 1

You can say 私が手紙を送ったのは彼だった.

This 私が is more or less important. If you omitted 私が, the sentence would become ambiguous:

手紙を送ったのは彼だった。

• He is the one who sent a letter (to someone).
• He is the one I sent a letter to.

The use of あげる cannot solve this type of ambiguity (手紙を送ってあげたのは彼だった is still ambiguous the same way). And even the following simple phrase is ambiguous in Japanese:

手紙を送った人

• a person who sent a letter (to someone) (=the sender)
• a person who someone sent a letter to (=the receiver)

Please read the following questions for details:

• Clarification about how 惚れた should be translated
• Relative Clause Ambiguous
• How is the subject of this subclause made clear?