No os olvidéis del salero ni del pimentero cuando pongáis la mesa.

Because Spanish very often uses olvidarse de + noun for to forget about / to fail to remember something.

So:

• olvidar algo = to forget something
• olvidarse de algo = to forget about something

In everyday Spanish, olvidarse de is extremely common and sounds very natural here. So No os olvidéis del salero is an idiomatic way to say Don’t forget the salt shaker.

You could also hear No olvidéis el salero, but no os olvidéis del salero is especially common in speech.

Because negative commands in Spanish use the present subjunctive, not the regular imperative form.

For vosotros, the pattern is:

• affirmative command: olvidad
• negative command: no olvidéis

Since this verb is being used as olvidarse, you also need the pronoun os:

• affirmative: olvidaos
• negative: no os olvidéis

So No os olvidéis is the correct negative vosotros command.

In Spanish, object and reflexive pronouns go:

• before a negative command
• attached to the end of an affirmative command

So:

• No os olvidéis = negative command → pronoun before the verb
• Olvidaos = affirmative command → pronoun attached to the verb

That is why os appears before olvidéis here.

Os is the reflexive/object pronoun for vosotros, which is the informal plural you used in Spain.

So this sentence is speaking to more than one person in a familiar way, as if saying you all to friends, children, family members, classmates, etc.

Very roughly:

• te = you singular informal
• os = you plural informal in Spain
• se = you plural formal / third person style

So No os olvidéis... means the speaker is addressing you all in Peninsular Spanish.

Because del is the contraction of de + el.

Since the verb pattern is olvidarse de algo, you need de. And both salero and pimentero are masculine singular nouns, so they take el:

• de + el salero → del salero
• de + el pimentero → del pimentero

This contraction is mandatory in standard Spanish.

Spanish normally repeats the preposition/article combination with each noun in a structure like this.

So:

• del salero ni del pimentero

sounds more natural and complete than trying to use del only once for both.

English often avoids repetition more easily, but Spanish usually prefers to repeat these small words for clarity and rhythm.

Because ni means nor, and it fits naturally after a negative statement.

The structure is:

• No... ni... = not... nor...

So:

• No os olvidéis del salero ni del pimentero
• Don’t forget the salt shaker or the pepper shaker
• more literally: Don’t forget the salt shaker nor the pepper shaker

Using ni is the normal Spanish way to link the second item in a negative sentence like this.

Because after cuando, Spanish often uses the subjunctive when referring to a future action.

Here, the meaning is basically:

• when you set the table / when you go to set the table

That action has not happened yet, so Spanish uses:

• cuando pongáis la mesa

If it were about a habitual or general action, Spanish could use the indicative:

• Cuando ponéis la mesa, siempre falta algo.
• When you set the table, something is always missing.

So in this sentence, pongáis is used because the table-setting is a future situation relative to the command.

Literally, poner la mesa means to put the table, but idiomatically it means to set the table.

This is the normal Spanish expression for laying out plates, cutlery, glasses, and so on before eating.

So cuando pongáis la mesa is the natural Spanish way to say when you set the table.

Yes, it is clearly Peninsular Spanish because of os and the vosotros verb forms:

• olvidéis
• pongáis

In most of Latin America, the same idea would usually be expressed with ustedes instead:

• No se olviden del salero ni del pimentero cuando pongan la mesa.

So the original sentence is very natural for Spain.

If you were speaking to one person informally in Spain, you would use tú:

• No te olvides del salero ni del pimentero cuando pongas la mesa.

If you were speaking formally to one person, you would use usted:

• No se olvide del salero ni del pimentero cuando ponga la mesa.

So the original sentence is the plural informal version.