Difference between 'y' and 'e' // 'o' and 'u'

Hi there!
I was just wondering: what's the (grammatical) difference between the adverbs {'y' and 'e' } & {'o' and 'u'}? (I suppose it has something to do with the first characters of the next word ..^o) )
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To avoid oral melting the two vowels you change i.e. from "y" to "e" like:

1...derecha E izquierda... instead of 2...derecha Y izquierda...

1. is correct, it sounds clearer.

 

Ohw ok, that's clear!

Thus, it's 'e' if the next word starts with an 'i' or 'hi', and (in case of the 'o/u') 'u' if the next word starts with an 'o' or 'ho' ..

Thanks!