Chainy:

erinja:"they" and "them"

As this distinction exists for such a word, I wonder if English speakers are more likely to remember 'iliN', but in other cases where no apparent change takes place in English, they also forget in Esperanto?

The reason why I'm wondering this is that as a lingvohelpanto here I sometimes have slavic students and I've noticed that even they can forget the accusative, particularly when a word in their own language happens to have the same ending in the accusative as in the nominative...

So perhaps it's a case of people following patterns in their native language, rather than understanding the actual concept of the accusative. Slavic students can get away with such an approach to a much greater extent as they have many more words that change in the accusative form.

This is an intriguing question worthy of study, in fact I virtually never forget the accusative with kiu(n) (German wer - wen) but sometimes with kio(n) (German both was).

There is only one idea you need to get your head round for all uses of '-n'. And that is how does the addition of '-n' change the meaning. In many uses it serves as a precizigiilo - it ties down the meaning or limits the interpretation.