In Korean, the particles "이" and "가" are used to indicate the subject of a sentence. "이" is attached to nouns that end in a consonant, while "가" is used with nouns that end in a vowel. For example, in the sentence "사람이 웃어요" (The person laughs), "사람" (person) ends with a consonant, so "이" is used. These particles help clarify who or what is performing the action in a sentence. They are essential for understanding the structure of Korean sentences. For instance, in "고양이가 자고 있어요" (The cat is sleeping), "고양이" (cat) ends with a vowel, so "가" is used.