Learn Hardcore Portuguese (Portugal): Por favor, lembra-te de apagar a luz antes de sair. - Please remember to turn off the light before leaving.

lembrar algo a alguém = to remind someone of something
- Lembra-me disso. = Remind me of that.
lembrar-se de algo / de fazer algo = to remember something / to remember to do something

In your sentence, the meaning is “remember to turn off the light”, so Portuguese uses the reflexive form lembrar-se de:

lembrar-te de = for you (tu) to remember to…

So lembra-te de apagar a luz literally is “remember yourself to turn off the light”, the normal way to say “remember to turn off the light” in European Portuguese.

What exactly does te mean in lembra-te?

Te is the unstressed object pronoun for “you” (informal singular, tu).

In reflexive verbs like lembrar-se, it shows who is doing the remembering:

lembrar-me = I remember / for me to remember
lembrar-te = you (tu) remember / for you to remember
lembrar-se = he, she, you-formal, they remember
lembrar-nos = we remember

In Por favor, lembra-te…, te refers to the person you are talking to informally (a friend, child, etc.).

What grammatical form is lembra-te? Is it a tense?

Lembra-te is the affirmative imperative (a command or request) for tu (informal singular you) of the reflexive verb lembrar-se.

Formation (for tu in regular speech):

Take the present indicative: tu lembras
Drop the final -s: lembra
Attach the reflexive pronoun with a hyphen: lembra-te

So lembra-te = “remember (you)!”

Why is the pronoun attached with a hyphen (lembra-te) instead of before the verb?

In European Portuguese, pronoun placement changes depending on the structure:

Affirmative imperative: pronoun goes after the verb with a hyphen (enclisis)
- Lembra-te de apagar a luz.
Negative imperative (and some other tenses): pronoun goes before the verb (proclisis)
- Não te lembres disso. = Don’t remember that.

So the hyphen in lembra-te is normal for affirmative commands like this one.

How would the sentence change if I’m talking to more than one person?

If you are addressing more than one person (you plural, vocês), you use lembrem-se:

Por favor, lembrem-se de apagar a luz antes de sair.
= Please remember to turn off the light before leaving.

Details:

lembrem-se is the imperative for vocês of lembrar-se.
The rest of the sentence stays the same.

How would I say this politely to a stranger or someone I address formally?

In formal European Portuguese (to o senhor / a senhora), you normally use lembre-se or, even more commonly, não se esqueça:

Por favor, lembre-se de apagar a luz antes de sair.
Por favor, não se esqueça de apagar a luz antes de sair.

Both are polite. Não se esqueça (“don’t forget”) is very frequent in everyday speech.

Why is it lembrar-te de apagar and not just lembrar-te apagar?

With lembrar-se, Portuguese requires the preposition de before a noun or an infinitive:

lembrar-se de algo = to remember something
lembrar-se de fazer algo = to remember to do something

So:

lembrar-te de apagar a luz = to remember to turn off the light

Leaving out de (✗ lembrar-te apagar a luz) is ungrammatical in European Portuguese.

Why is it a luz and not just luz?

Portuguese normally uses the definite article (a, o, as, os) where English often doesn’t.

Here, a luz means “the light”, referring to a specific light in the room, corridor, house, etc. In this kind of daily instruction, you almost always say:

apagar a luz = turn off the light

Saying ✗ apagar luz sounds unnatural in European Portuguese in this context.

What does antes de sair literally mean, and who is the one “leaving”?

Antes de sair literally is:

antes = before
de = of / to (here just a linking preposition)
sair = to leave / to go out (infinitive)

So: “before leaving”.

In this sentence, the understood subject of sair is the same person you’re talking to (the tu of lembra-te), even though it isn’t repeated:

You (tu) → remember to turn off the light before you leave.

Portuguese can omit the subject here because it’s clear from context.

Can I say antes de saíres instead of antes de sair?

Yes, in European Portuguese you can, and many speakers do:

Por favor, lembra-te de apagar a luz antes de saíres.

Antes de saíres uses the personal infinitive, which marks the subject (tu) explicitly:

antes de sair = before leaving (subject understood from context)
antes de saíres = before you leave (subject marked by -es)

Both are grammatically correct. Antes de sair is slightly more neutral and general; antes de saíres makes it extra clear that you are the one leaving.

How would I say “Please don’t forget to turn off the light before leaving” in a similar style?

A natural equivalent with “don’t forget” is:

Por favor, não te esqueças de apagar a luz antes de sair.

Notes:

não te esqueças = don’t forget (tu, informal)
Structure after it (de apagar a luz antes de sair) is the same as in the original sentence.

Where can por favor go in the sentence? Does its position change the meaning?

You can move por favor around without changing the basic meaning. Common options:

Por favor, lembra-te de apagar a luz antes de sair.
Lembra-te de apagar a luz antes de sair, por favor.
Lembra-te, por favor, de apagar a luz antes de sair.

All are understood as polite requests. Changing the position only adds a slight difference in emphasis or rhythm, not in politeness level.

Is this sentence the same in Brazilian Portuguese?

It’s very similar, but Brazilians often drop the reflexive in informal speech:

(BR informal) Por favor, lembra de apagar a luz antes de sair.

In European Portuguese, especially in more careful speech, lembrar-se de (reflexive) is preferred:

(PT) Por favor, lembra-te de apagar a luz antes de sair.

So if you’re aiming for Portuguese from Portugal, keep the reflexive form lembrar-te de here.