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You determine whether to use ㄹ 거예요 or 을 거예요 based on the last vowel of the verb stem. You use ㄹ 거예요 after verb stems that end with a vowel, such as 자다 (jada, meaning “to sleep”). You use 을 거예요 after verb stems that end with a consonant, such as 먹다 (meokda, meaning “to eat”).