Learn Spanish (Spain): De cambiar el protocolo, avisad al personal. - If the protocol changes, notify the staff.

Using de directly followed by an infinitive is a concise way to express a condition. It means the exact same thing as si cambia (if it changes). This structure is highly common in formal instructions, procedures, or rules.

Is the subject of the infinitive always the same as the main clause when using the de + infinitive structure?

Not necessarily! In this sentence, the protocol is the implied subject of cambiar (if the protocol changes), while the subject of avisad is vosotros (you all). As long as the context makes it clear, the subjects of the two halves of the sentence can be different.

When should I choose de + infinitive instead of the standard si for 'if'?

You will mostly see de + infinitive in written instructions, warnings, or formal business contexts. It works perfectly here because it is dictating a procedural rule. For everyday spoken Spanish, si + conjugated verb remains the most natural and common choice.

What verb form is avisad?

It is the affirmative imperative (command) form for vosotros (you all, informal). In Spanish from Spain, you form this simply by replacing the final -r of the infinitive avisar with a -d.

Why is there an before ?