Learn Russian: Не нажимай эту кнопку!

In Russian, when you tell someone not to do something (a negative command), you almost always use the imperfective aspect. Even though pressing a button is a quick, one-time action, adding не makes it a strict prohibition, so we use the imperfective нажимай.

How do I form this command from the dictionary word нажимать?

You start with the 'they' form of the verb, which is нажимают (they press). Drop the -ют ending and add -й. This gives you the informal singular command нажимай. If you were warning multiple people or someone you speak to formally, you would add -те to make it Не нажимайте!

Why does it say эту кнопку instead of эта кнопка?

The button is the direct object receiving the action of being pressed, so it must be in the accusative case. For feminine words, the -а endings change to -у, making эта кнопка become эту кнопку.

How would I say the opposite: 'Press this button!'?

For an affirmative command to complete a one-time action, you switch to the perfective aspect. The perfective verb is нажать, and its imperative form is нажми. So you would say: Нажми эту кнопку!