As I don't know his address, I can't write to him.
→If I knew his address, I could write to him.
仮定法ですね。「もし知ってたら手紙書けたのになぁ」です。もしもっと勉強してたらこの問題も解けたのになぁ(笑) If I studied more, I could answer the question.ですね(^.^)
それともう一問の方は
They insisted that we should clean the room.を単文に書き換えてね…ですが
They insisted on us(our) cleaning the room.ですよ〜♪ 単文は主述関係が文の中に一つなので書き換え前のthat節以降を分詞を使って書き換えてくれたらよかったんですけどね。