Learn Spanish (Latin America): Si presionas el botón rojo, enciendes la pantalla. - If you press the red button, you turn on the screen.

When talking about facts or things that happen immediately, Spanish often uses the present tense in both halves of a si (if) sentence. Since pressing the button immediately turns on the screen, both presionas and enciendes are in the present tense.

Why is the verb spelled enciendes instead of encendes?

The verb encender (to turn on) is a stem-changing verb. In the present tense, the second e changes to ie for most forms, including the tú (you) form, giving us enciendes.

Should the word Si have an accent mark?

No, it shouldn't. Without an accent mark, si means 'if'. With an accent mark, sí means 'yes'.

Could I include the word tú (you) in this sentence?

Yes, you could say Si tú presionas..., but it is optional and usually left out. The -s endings on presionas and enciendes already make it perfectly clear that you are talking to tú.