I think I pretty much get this. Masculine nouns you add an 'e', (or drop soft sign then add an 'e'). This is with the exception of a few such as саду.
For female/neuter nouns, drop the vowel and add an 'e'. If its a soft sign remove it and add an и. This is where my confusion comes in below - this word goes from
упражнение to в упражнении. Now this i understand as it is because и was the second to last letter so the final letter also changes. So why does this word not change? Огайо should this not change to Огайи??
Thanks if someone can clear this up.
Thanks
Andy