I've found a fantastic way of explaining the passé composse: It's not just the past tense. It's "a speshul kind of past tense or else it wouldn't be called passe compossé! Because God knows that grammar doesn't have enough useless classifications.
In all seriousness, it's not just 'past tense'. It's like 'past tense but when you're conjugating verbs in a strange, strange ways that they where never meant to be conjugated and oh my god what is that!' or something."