I am trying to find a logical way to explain how/when to use "done" vs "did" and "seen" vs "saw".  The person I'm coaching uses the terms in the following manner and I just don't want to correct him, but would also like to give him the rationale so that he understands how to use these terms in the future.

"I done the letter and sent it on Monday".
"I seen a boy running down the street" or "I seen that movie".

My pupil is a mature person who is generally bright, but poor usage habits are hard to break and even harder to explain!

Thanks!