Some sentence structures, which may feel natural while you’re writing them, are just naturally confusing. Here’s an example of one I notice often:
Don’t press the button on the controller until the green light turns on.
This is a garden path sentence, which requires a reader to backtrack to successfully parse the meaning. It initially appears as though the sentence presents a thing you ought not do. But when you read to the end of the sentence, it resolves as a thing you ought to do after a condition has been met. While careful readers will understand the meaning, non-native English readers and expert skimmers alike may understandably misunderstand it.
This is a fixable problem, however. You might be tempted to remove only the don’t from the sentence, like so:
Press the button on the controller when the green light turns on.
That’s somewhat better. A bigger proportion of readers are likely to understand that the button should be pressed, but now they may not notice that there’s a condition to be satisfied before pressing the button. Instead, I recommend this approach:
When the green light turns on, press the button on the controller.
Not only does adhering to the common if-then conditional structure appeal to the logical part of my brain, this sentence structure makes an implication to the reader: wait before acting. This pattern is less likely to be difficult for readers to parse than the first example and makes the existence of the requirement more obvious to all readers than both previous examples, even those skimming from the middle of the sentence.
The only time it’s appropriate to go negative is when the reader should always avoid the instructed activity, or when doing the activity would represent an exceptional case. When giving a negative instruction, start with the strongest appropriate words, like these examples:
Never press the red button. Avoid getting the device wet. Do not press the red button unless instructed to do so by a technician.
It’s easy to get wrapped up in the higher-level concerns of your writing, like scope and organization, but don’t forget these sentence-level details. They’re easy to miss, but important for making your prose understandable and helpful.