Starting with a negative for dramatic effect
The formal word-order swap that follows a fronted negative phrase, and why it makes writing sound emphatic.
Not only did the company miss its targets, but it also lost its two biggest clients.
When a negative or restrictive adverbial opens the sentence for emphasis, the subject and the auxiliary swap places, exactly as they would in a question. If the verb has no auxiliary of its own, the dummy auxiliary steps in to carry the tense and the main verb returns to its base form. The inversion applies only in the clause the fronted phrase introduces; the rest of the sentence keeps normal word order. The effect is formal and dramatic, at home in speeches, formal writing and literary prose.
The first trap is halfway inversion: fronting the negative phrase but leaving subject and verb in normal order, which reads as a plain error rather than emphasis. The second is forgetting the dummy auxiliary when the verb is a simple present or past - those forms have no auxiliary of their own to move, so one must be supplied.