YOU SAID:

if i had not have been there the day before the last day of school then he would be here today

INTO JAPANESE

もし私が学校最終日の前日にそこにいなかったら、彼は今日ここにいただろう

BACK INTO ENGLISH

If I hadn't been there the day before the last day of school, he would be here today.

INTO JAPANESE

もし私が学校の最終日の前日にそこにいなかったら、彼は今日ここにいるでしょう。

BACK INTO ENGLISH

If I hadn't been there the day before the last day of school, he would be here today.