Breakdown of Debes poner el pestillo en el baño, no vaya a ser que la niña entre.
la niña
the girl
en
in / on / by (means of transport) / about (after pensar)
el baño
the bathroom
poner
to put / to set (the table) / to make (an excuse) / to give, to impose (a fine, a penalty)
Questions & Answers about Debes poner el pestillo en el baño, no vaya a ser que la niña entre.
Why use the long phrase no vaya a ser que instead of just translating "just in case"?
No vaya a ser que (and its shorter cousin no sea que) is a very natural Spanish expression used for preventive actions—doing something so that a bad or unwanted thing doesn't happen. It literally translates to "it is not going to be that..." but means "lest" or "just in case".
Why must we use the subjunctive entre after no vaya a ser que?
Clauses that express a preventive purpose (doing X so that Y doesn't happen) always take the subjunctive. This is because the second action (entre) is a hypothetical, unconfirmed event that you are actively trying to prevent from becoming reality.
Could I use por si acaso in this sentence instead?
Absolutely. You could say ...por si acaso la niña entra. Notice a key grammar difference, though: por si acaso is usually followed by the indicative (entra), whereas no vaya a ser que always demands the subjunctive (entre).
Is poner el pestillo the normal way to say "lock the door" in Spain?