Only three particles have two versions for the ease of pronunciation. -는/은 -는 after vowels: 나는, 너는, 바다는, 비둘기는, 한국어는 -은 after consonants: 가슴은, 하늘은, 꽃은, 말은, 땅은 -를/을 -를 after vowels: 나를, 너를, 바다를, 비둘기를, 한국어를 -을 after consonants: 가슴을, 하늘을, 꽃을, 말을, 땅을 -가/이 -가 after vowels: 내가 (this is irregular, it's never 나가, always 내가), 네가 (also irregular, never 너가), 바다가, 비둘기가, 한국어가 -이 after consonants: 가슴이, 하늘이, 꽃이, 말이, 땅이 But all other particles like -에, -에서, -의, -도, -조차, etc. only have one form. 나에, 나에서, 나의, 나도, 나조차 가슴에, 가슴에서, 가슴의, 가슴도, 가슴조차

Hyejeoung is right. I will add one thing and liken it to "a" and "an" in English. Same concept (ease of pronunciation)