This is an important distinction and the following example will help to explain it.

A little history about this particular plant may help to explain the reason for the emotional response.

Perhaps that would help to explain what he had first thought to be her irrational action.

That helped to explain why the two worked so well together.

Probably this helps to explain the success his first book met with all over the world.

Perhaps that would help to explain my driver's obvious fear.

A recent study helps to explain why this problem is so common.

That might help to explain why so many girls become involved with inappropriate young men.

But I think it will help to explain a lot of this.

Taken together, they help to explain why the student movement came to grief.