It is essential for young people to have the ability to plan and organize.
If a high school student is about to leave school soon, it is necessary for him to have a most suitable plan before the graduation ceremony begins.
Teachers should give students homework everyday.
If teacher had been checking the students' progress with daily homework assignments, they might not have failed the test.
if sb had done, sb might/would have done
Traveling alone is better than travelling with a companion.
But for the assistance from a companion, travelers would be exposed to more dangers on the way.
现在或将来的情况：but for sb/sth, sb would do sth
过去的情况： but for sb/sth, sb would have done