Understanding tenses can be challenging, especially when you hear a long string of helping verbs. For example, imagine how difficult it is for someone to understand the following sentence the first time they hear these tenses used.
We'll have finished the report by the time the conference begins.
Or this ...
We had been waiting for more than three hours by the time he finally arrived.
It seems clear if you speak English, but rather strange if it's your first time! I've found that providing visual clues can really help students understand how all of the tenses work together. There's the visual tenses gallery providing a timeline for each use of a tense with an example. For a broader overview, try the tense timeline that arranges all the tenses along a visual timeline from past to present.