(1) Read the report if you need the information.
(2) I will read the report if I need the information.
(3) I would read the report if I needed the information.
(4) I would have read the report if I had needed the information.
1,2は現実
3,4は非現実（過去又は未来もしもそうなったら）
1,2,3は現在又は未来
4は過去
