How to explain grammar to a 12 years old...

I find that while doing high intensity courses during the summer (by high intensity I mean that in 8 or 9 lessons the student ought to have covered basic questions, conjugating regular and irregular verbs and have acquired a certain amount of vocabulary. ) While most of the class and rewards are done in Spanish, switching to English for that one second may help.
Here is an example of how I dissect verb conjugating to explain it to a 12 years old.
By using questions and a rather straight forward POINT BY POINT explanation of the HOW rather than WHY verbs are irregular I usually get excellent lasting results.
To design this slide I used Smartboard Notebook software,  this slide came after an explanation of the three groups of verbs, the subject pronouns used in Spanish and a slide on comparing the verb Estar vs the irregular pattern. 
A well thought out visual can cut many corners with this type of student ( I'm talking a bout Middle schoolers, but infographics are great for ANY age!). It is a ton of into to take in and so having clearly labelled and laid out slides that are colorful and engaging always helps. 
I also tell my students who also learn English grammar, of the connections between the languages,
the historical reason in French for the circumflex accent too. They find these stories engaging and we are often off to a good start.