She (had taught, taught) in an elementary school before she became a professor

In my grammar book, the answer is only "had taught" in this sentence, but isn't "taught" also possible as time sequence is evident due to "before"? Are both correct or only one?
ex)She (had taught, taught) in an elementary school before she became a professor.

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Originally Posted by keannu

In my grammar book, the answer is only "had taught" in this sentence, but isn't "taught" also possible as time sequence is evident due to "before"? Yes
Are both correct or only one? Both

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When the sequence of events is clear, we frequently use the past simple.