Substitude the underlined word by a direct object pronoun.J'achète une robe pour ma sœur.

“Une robe” is feminine singular, so its direct object pronoun is “la.” In French, the direct object pronoun goes before the verb, and “la” shortens to “l’” before the vowel sound in “achète.” Therefore, the correct sentence is “Je l’achète pour ma sœur.”

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