In grammar, a sentence fragment refers to a set of words that lack one or more elements to be a complete sentence. In the case of the sentence "When we stopped by the store during our lunch break" this is a sentence fragment because this is a dependent clause, this means this sentence needs from another to be completed and due to this, a context is given "when we stopped" but not the action that occurred in that context or what happened as the word "when" implies something occurred at that moment. For example "When we stopped by the store during our lunch break. We saw Carla". Therefore, the type of error is a fragment.
