What should I tell him?

A teacher – English not Chinese – today asked me if I could give him a quick run down of the English tense system, as he isn't confident of his grammatical knowledge. This isn't at all uncommon here. In fact, as someone who actually has more than the most basic grammar knowledge, I am very much the exception rather than the rule when it comes to foreign teachers.

Trouble is I don't really know where to go with this.

I could explain to him in detail about tense, aspect, voice and mood. I could show him that English really has only two tenses, as such – present and past – and that what we commonly think of as the future is really formed with the modal "will". I could explain why anybody who says the passive must not be used is an idiot (though I'd have to get that explanation of "voice" done first.)

The trouble is, I'm a teacher here myself and I know that this isn't what he needs. I don't even know how I should tackle these things with my students. The Chinese grammar books are all at odds with my understanding of this area and the important thing to a student here isn't that he is any good at English, it's that he can pass his exam and go to University.
And I've gone on at length before about why a good knowledge of English grammar actively works against Chinese students in exams.

Imagine how much harder it will be for a teacher that got his grammar second-hand from me to make the call on what to pass on to the students.

Of course I could teach it to him as I learned it in school - where pretty much everything I was ever told was wrong. I could teach that English has twelve tenses (which is what my teachers told me) – the various combinations of what I now know to be present, past, future (modal will) with continuous/simple/perfect aspects. Of course that would be ignoring mood (no great loss there) and voice (a rather more substantial loss).

Or I could just do a "lies to children" version giving him just the information that he needs to pass on to his students to get them through the exams.

He's not a stupid guy, but he does suffer from having gone to school in England in that period when the powers that be had decided that grammar should no longer be taught, so he lacks even the very basic stuff like a broad understanding of what a noun or a verb is. He's actually a pretty good teacher for the job we do. He gives the students interesting discussion tasks, has a great connection with them and works hard in the classroom. His need is more perceived than actual, as we are discouraged from teaching grammar. All the same grammar questions come up – especially with private students – so he wants to understand well enough to answer them.

But what is well enough?

What would you tell him?