Não mexas no interruptor, por favor.

Here mexer does not just mean to move in a literal sense. In the expression mexer em something, it often means:

• to touch
• to fiddle with
• to mess with
• to tamper with

So Não mexas no interruptor sounds like telling someone not to touch or fiddle with the switch.

Because mexer in this sense normally takes the preposition em:

• mexer em algo = to touch / meddle with something

Then em + o contracts to no:

• em o interruptor → no interruptor

This is very common in Portuguese. Similar contractions are:

• em + a → na
• em + os → nos
• em + as → nas

So no interruptor is grammatically required here.

Mexas is the present subjunctive form for tu of the verb mexer.

Portuguese uses the present subjunctive for negative commands:

• Não mexas = negative command to tu

Compare:

• Mexe! = positive command to tu
• Não mexas! = negative command to tu

So even though this is a command, the negative form does not use the normal affirmative imperative form.

It is addressed to one person, and specifically in the informal singular form: tu.

The subject pronoun is not written, but it is understood:

• (Tu) não mexas no interruptor, por favor.

This is very normal in Portuguese, especially because verb endings already show who is being addressed.

Yes. In European Portuguese, using tu like this is very normal in informal situations, especially with:

• family
• friends
• children
• people you know well

So Não mexas no interruptor, por favor sounds natural in Portugal if you are speaking informally to one person.

You would change the verb form.

Formal singular

• Não mexa no interruptor, por favor.

This uses the 3rd person singular form, the one used for você or formal address.

Plural

• Não mexam no interruptor, por favor.

This is for vocês.

In Portugal, formal language often avoids the pronoun itself, so you may hear Não mexa... without você.

Because in standard Portuguese, the negative command for tu uses the present subjunctive, not the affirmative imperative form.

So:

• Mexe! = correct affirmative command
• Não mexas! = correct negative command
• Não mexe! = not standard for tu

That is one of the most important imperative patterns to learn in Portuguese.

Yes, you could, and it would also sound natural.

There is a small difference in nuance:

• tocar = to touch
• mexer em = to touch, handle, fiddle with, mess with

So:

• Não toques no interruptor focuses more directly on touching
• Não mexas no interruptor can suggest don’t touch it / don’t fiddle with it / don’t mess with it

In many situations, mexer sounds especially natural when warning someone not to interfere with an object.

Interruptor usually means a switch, especially an electrical one such as a wall switch.

In everyday use in Portugal, it often refers to something like a light switch.

It is worth not confusing it with:

• tomada = socket / power outlet
• botão = button

So if you mean the wall switch, interruptor is the right word.

The tilde in não shows nasalisation. It does not work like an English-style stress mark.

So não is not pronounced like now or no. It has a nasal sound, roughly similar to nãw with the vowel resonating through the nose.

A few pronunciation notes for the whole sentence in European Portuguese:

• não = nasal
• x in mexas sounds like sh
• final -as in European Portuguese is reduced, roughly like -ush
• interruptor is stressed on the last syllable: interrupTOR

So mexas sounds roughly like MEH-shush in European Portuguese.

Yes. Por favor is flexible.

All of these are natural:

• Não mexas no interruptor, por favor.
• Por favor, não mexas no interruptor.
• Não mexas, por favor, no interruptor.

Putting por favor at the end is very common and sounds polite without being too formal.