No ho oblidis: abans de bufar, hem d'apagar la llum.

Because this is a negative command: Don’t forget it.

In Catalan, negative commands are built with no + verb, and object pronouns usually go before the verb:

• No ho oblidis = don’t forget it

With a positive command, the pronoun usually goes after the verb:

• Oblida-ho = forget it

So the change is not random; it is part of how Catalan handles commands and pronouns.

Because negative commands in Catalan use the present subjunctive form.

For tu:

• positive command: oblida
• negative command: no oblidis

So:

• Oblida-ho = forget it
• No ho oblidis = don’t forget it

This is one of the most important command patterns to learn in Catalan.

Ho is a neuter direct object pronoun. It often means it, this, that, or even the whole idea we’re talking about.

In this sentence, ho probably refers to the reminder or instruction as a whole, not to a specific masculine or feminine noun.

That is why ho is very natural here:

• No ho oblidis = don’t forget that / don’t forget it

If you were replacing a specific noun, you might use another pronoun instead:

• el for a masculine singular noun
• la for a feminine singular noun

Yes, that is also possible, because Catalan often uses the pronominal structure oblidar-se de.

So you can have:

• No ho oblidis
• No te n'oblidis

Both can mean don’t forget it / don’t forget about it, but they are built differently:

• oblidar alguna cosa = to forget something
• oblidar-se de alguna cosa = to forget about something

In your sentence, No ho oblidis is completely correct and straightforward.

Because when abans is followed by an infinitive, Catalan normally uses de:

• abans de + infinitive

So:

• abans de bufar = before blowing

More examples:

• abans de sortir = before leaving
• abans de menjar = before eating

If you use a full clause instead of an infinitive, Catalan usually uses abans que:

• abans que bufis = before you blow

Bufar means to blow.

In this context, it most likely means to blow on / blow out candles, as in a birthday-cake situation.

A useful distinction is:

• bufar = to blow
• apagar = to turn off / extinguish

So bufar is the action of blowing, while apagar la llum is the action of switching off the light.

It means we have to turn off or we must turn off.

This uses the Catalan structure:

• haver de + infinitive = to have to

So:

• hem de = we have to
• apagar = turn off / switch off / extinguish

Together:

• hem d'apagar = we have to turn off

This is a very common way to express obligation in Catalan.

Because de becomes d' before a word beginning with a vowel or mute h.

So:

• de apagar becomes d'apagar
• de obrir becomes d'obrir

This is just normal Catalan spelling and pronunciation.

In your sentence:

• hem de apagar would be wrong
• hem d'apagar is correct

Because la llum often works like the light in a general sense, meaning the lighting in the room.

So:

• apagar la llum = turn off the light

Even if a room has several bulbs or lamps, Catalan very often uses the singular for the general idea of the light being on or off.

This is the normal everyday expression.

Yes. It is the normal and idiomatic expression.

Common pairs are:

• encendre la llum = turn on the light
• apagar la llum = turn off the light

So if you want the natural everyday Catalan phrase, apagar la llum is exactly what you want.

The colon introduces the reminder or explanation that follows.

So the structure is something like:

• Don’t forget this: before blowing, we have to turn off the light.

It works very much like English in that kind of sentence. The first part gets the listener’s attention, and the colon introduces the important instruction.