Learn Russian: Нажми красную кнопку.

In Russian, positive commands (telling someone to actually do a specific action) usually use the perfective aspect. This focuses on completing the action to get a result—in this case, the button successfully being pressed.

How would I say "Don't press the red button!" based on the lesson rules?

For negative commands (telling someone NOT to do something), Russian almost always switches to the imperfective aspect. So, you would use the imperfective verb нажимать and say Не нажимай красную кнопку!

Why is it красную кнопку instead of the dictionary form красная кнопка?

The button is the direct object of the sentence because it is the thing receiving the action of being pressed. That means it must go into the accusative case. For feminine singular nouns and adjectives, the dictionary -ая ending changes to -ую, and the -а ending changes to -у.

Is нажми formal or informal?

Нажми is the informal, singular command used for someone you would call ты. If you were talking to someone formally (using Вы) or to a group of people, you would just add -те to the end, making it Нажмите красную кнопку.