My daughter was finding it difficult to remember the plural ie. bahu bachan for Hindi words ending with "badi ee".
So I told her a story.
Whenever you have to write the "bahu wachan" of words ending in "badi ee", think of "mummy" in Hindi and how it is written. One mummy is thin enough for the "ee" ki matra to be on the right side. When there are mummies, then there is no space for the "ee" ki matra. Hence the "badi ee" ki matra shofts to the left and becomes "chotti ee". and to support it we have "ian".