-과 and -와 is a same connecting particle. -과 is attached to nouns in which the last letter is a consonant and -와 is attached to nouns in which the last letter is a vowel The same rule applies to -은/-는, -을/-를, -이/-가 etc. The original form is 와. ㄱ sound is added for pronouncing fluently after a word that end in a consonant. Similar transition can be found in Mongolian language also. ex) 책 + 와 => 책과 친구 + 와 => 친구와

-와 is added to words that end in a vowel and -과 is added to words that end in a consonant (받침).