When teaching grammar, stick with the uncontroversial stuff...

I was in a grammar class for my Gaelic course today, and we were looking at noun declensions.  For one set of questions, we were using the word fàinne (ring).  Unfortunately, this word is masculine in some dialects and feminine in others.  The question stated that the word was masculine, but my partner for the exercise (a native speaker) has used it all her life as a feminine word.  She declined it perfectly correctly in each case -- as a feminine word.  The explicit instruction to decline it as masculine was ignored because she already has 100% intuitive command of the word.

Correct completion of the task therefore required that she stop dealing with the words as "language" and start thinking of them as some kind of mechanical logic puzzle.

The problem with the task is that it became counter-intuitive.  When teaching grammar, we need to employ as much pre-existing knowledge as possible.  Grammar teaching for natives has to start with forms they know, because you are not actually teaching "grammar", you are teaching "grammar awareness", and that simply means making them consciously aware of things they already know intuitively.

So you have to pick the most uncontroversial examples, the most universal and unchanging.

Work with your students, not against them.