Если часть предложения с if в начале предложения, после нее ставим запятую, если во второй половине, запятую не ставим.
If...., ....
.... if ...
If people think more about ecology, we'll be a healthy.
We`ll have no shortage of food when we'll preserve the nature.
We finish this task when we solve this problem.
We'll have pure water if we use filters.
We shall survive if there is no a global heating.