Is there a rule behind why some gerrunds take -yendo as an ending?

Question

I am just wondering why some -ir and -erverbs use -iendo and the other take -yendo.  Does it have to do with the double vowel?  Like -uir -aer -eer?

Accepted Answer

Hello there KT ... Yes, that does indeed usually occur in '-ir' and '-er' verbs with a double vowel: "oír" is another example - but it does not happen with "reír" because of the e>i stem-change [nor of course with sonreír], so their gerunds are [slightly] irregular, i.e. 'riendo'... "Freír" follows this pattern too. // [Without the stem-change, they might have become 'reyendo', 'freyendo'].
In those last examples ['-eír' verbs], an "iy" combination would be regarded as clumsy and unnecessary - so it is avoided. // For the same reason, '-iar' verbs, too, would never form gerunds like 'limpiyando' or cambiyando!
In fact [as you imply], all '-ar' verbs do not insert a 'y' to break the double vowel, e.g.,'crear', 'pelear', 'actuar', 'averiguar'... [> creando, actuando, etc. etc.]