“Do not leave the form unsigned unless approval is unnecessary.” A reader may understand each word and still pause over what to do next. When instructions stack negatives and exceptions, revision should begin with the decision the reader must make.
Translate the sentence into actions
For the invented form example, first establish the intended rule: when approval is required, sign the form; otherwise, follow the process for forms that need no approval. Do not simplify the wording before confirming that interpretation. A smoother sentence that reverses a condition is a worse instruction.
Put the main case before the exception
Try “Sign forms that require approval.” Then explain the exception in a separate sentence if the user actually needs it. Naming the action lets the reader see the usual path before considering the boundary case.
Some prohibitions need to remain prohibitions. “Do not submit personal data” should not be softened into an invitation to choose unspecified alternative information. Clarity comes from preserving the rule while reducing the effort needed to understand it.
Test with two concrete situations
Invent one case where the condition applies and one where it does not. Read the revised wording as if you were the person in each situation. Can you decide what to do without mentally undoing two or three negatives?
This is a focused use of testing a claim with an example. Before polishing the tone, write down the action, the condition and the exception in plain terms. Once those agree with the intended meaning, the final instruction can become shorter without becoming careless.