What is a negative prompt, exactly?
A negative prompt is a separate instruction channel that biases the model away from the concepts you list. It does not delete pixels after the fact — it shifts the generation so the listed concepts are less likely to appear. That distinction matters: a negative prompt nudges probabilities, it doesn't guarantee total absence, so strong or repeated terms work better than a single weak mention.
The reason you can't always just say "no X" in the positive prompt is that image models attend to the concept you mention regardless of the word "no" — mentioning "no rain" can still surface rain. Routing exclusions through the dedicated negative channel (the negative-prompt field in Stable Diffusion, or `--no` in Midjourney) is what makes them reliable. For positive-prompt structure, the DALL-E Prompt Creator and Midjourney Prompt Builder are good companions.