There are two variants of the indefinite article: a and an. You use “a” when the word starts with a consonant sound and “an” when the word starts with a vowel sound.
The word “historic” can be pronounced with a hard “h” (consonant sound) or silent “h” (vowel sound). You should choose the indefinite article that fits your pronunciation.
I watched a historic movie.
I watched an historic movie.
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