If you got a letter on your birthday from your friend, what kind of things would make you feel happy when reading it?
ご質問ありがとうございます。
"If you got a letter on your birthday from your friend,"＝「もし、あなたのお友達から誕生日に手紙をもらったとしたら、」
"what kind of things would make you feel happy when reading it?"＝「その手紙を読むときに、どのような内容なら幸せを感じますか？」
ご参考までに。
If you were to receive a birthday card from someone, what would you like them to write to you?
If someone were to give you a birthday card, what would you like them to write in it?
この場合、次のように先生に聞くと良いですよ。
ーIf you were to receive a birthday card from someone, what would you like them to write to you?
「誰かから誕生日カードを受け取るとしたら、その人になんて書いてもらいたいですか？」
「何と書いてもらいたいですか」ということは「何と書いてあったら嬉しいですか」とも言えますね。
ーIf someone were to give you a birthday card, what would you like them to write in it?
「誰かがあなたに誕生日カードをあげるとしたら、その人になんて書いて欲しいですか？」
このように仮定法、If you were to ..., what would you ...? の形にすると良いでしょう。
ご参考まで！
アメリカ合衆国 
カナダ 
