Não te rias de mim, por favor.

Because this is a negative command.

In Portuguese, negative commands are not formed with the regular imperative. They use the present subjunctive instead.

So for tu:

• affirmative: Ri-te de mim.
• negative: Não te rias de mim.

So rias is the form you expect after não in this kind of sentence.

A small extra detail: rias is also spelled the same way as another verb form in Portuguese, but here the structure não + pronoun + verb clearly shows it is a negative command.

Because the verb here is being used in the expression rir-se de alguém, which means to laugh at someone or to make fun of someone.

So:

• rir = to laugh
• rir-se de alguém = to laugh at someone

In this sentence, te goes with tu. It is part of the verb construction in this meaning.

So Não te rias de mim is not literally about yourself in the English sense. It is just how Portuguese normally builds this idea.

Because não pulls the clitic pronoun before the verb.

In European Portuguese, little unstressed pronouns like me, te, se, lhe, nos often come after the verb, but some words force them to come before it. Negation is one of the main triggers.

Compare:

• Ri-te de mim. = affirmative command
• Não te rias de mim. = negative command

So não te rias is correct, while não rias-te is not.

Because after a preposition like de, Portuguese uses the stressed pronoun form.

That is why you get:

• de mim
• de ti
• de nós
• de ele/ela usually contracted differently in many contexts, depending on the structure

Here:

• mim is the correct pronoun after de
• eu is a subject pronoun, so de eu is wrong
• me is an unstressed clitic pronoun, not the form used after a preposition

So de mim is exactly what you should expect.

Because prepositions do not match word for word across languages.

English says laugh at someone. Portuguese says rir-se de alguém.

So it is better to learn the whole pattern:

• rir-se de alguém

rather than trying to translate each word separately.

It is informal singular.

The clues are:

• te
• rias

These point to tu, the informal singular you, which is very common in Portugal.

A more formal version, addressed to one person, would be:

• Não se ria de mim, por favor.

So if you are speaking to a friend, family member, classmate, or someone you address as tu, the original sentence fits well.

You may hear forms without the pronoun, especially in less careful speech or in other varieties of Portuguese, but for standard European Portuguese, the safer model for learners is:

• rir-se de alguém
• Não te rias de mim

So if you are learning Portuguese from Portugal, keeping te here is a good habit.

Por favor makes the sentence more polite and softer.

Without it:

• Não te rias de mim.

With it:

• Não te rias de mim, por favor.

The second version sounds more like a request and less abrupt.

Yes. It is flexible.

These are all natural:

• Não te rias de mim, por favor.
• Por favor, não te rias de mim.

The version with por favor at the end is very common and natural in everyday speech.

Because por favor works like a polite extra element added to the request, so it is often separated with a comma.

That comma helps show a small pause:

• Não te rias de mim, por favor.

You may sometimes see punctuation used a little loosely in informal writing, but with standard punctuation, the comma here is normal.