Grammar can be so messy, even the grammatical system of English, standard English in all its glory. On good days, we stand in awe at its magnificence, as when we look at the stars on a clear night; but on bad days it looks more like a cupboard full of the junk of history.
Seen through the eyes of a key stage 3 pupil, the junk may be intriguing, but it doesn't make sense; and what the pupil wants, above all, is sense. This is where the teacher comes in as the ultimate source of understanding.
For example, take the expression "if I were you". This is quite confusing for pupils who have been told that standard English demands "was" after "...". Does that mean that "if I were you" isn't standard English? Or is standard English actually less fussy than they thought? In fact, does anything go in standard English?
This particular debate may not arise in your classroom, but students should be encouraged to think critically about grammar as about everything else, and there are plenty of other exceptions to the rules of standard English, not least our National Anthem: "God save the Queen". Why not "saves"?
So how can you help them? One line would be to say that "if I were you" is just a fixed phrase, like "come what may". It's just a fossil from an earlier period. But that won't do because both "if" and "you" are flexible.
Your classroom reading could easily include a sentence such as "I hung as if I were annihilated" (H G Wells) or even "If I Were King" (A A Milne), which continues: I often wish I were a King, And then I could do anything.
If only I were King of Spain, I'd take my hat off in the rain.
Not great poetry, but powerful evidence that "I were" exists outside "if I were you".
By far the best way to explore and explain oddities such as "I were" is to collect examples of different uses (from the class, from a dictionary or from Google), and to let the class work out the rules.
This is an excellent thinking exercise for the class. For example, "I were" is only used after "if", "as if", "if only" and the verb "wish", and always describes a world which is possible but unreal - a useful tool for avoiding some ambiguities.
For example, a 2004 KS3 writer could satisfy the condition in: "if I was a baby in 1990", but not the one in: "if I were a baby in 1990".
This way lie both enlightenment and greater writing skill.