Precautionary clauses (no sea que)

Working in a blacksmith's forge requires a lot of safety precautions. When you take a specific action to prevent a negative or unwanted outcome, you use a precautionary clause. In English, we usually translate these as "just in case" or "lest".

no sea que

The phrase no sea que is a common way to express negative purpose. Because it describes a hypothetical event that you are actively trying to avoid, the verb that follows it must always be in the subjunctive.

Do not leave the metal there, hold it on the anvil, just in case it falls.
No dejes el metal ahí, sujétalo en el yunque, no sea que se caiga.
Always use glasses when forging, just in case a spark damages your eye.
Siempre usa los lentes al forjar, no sea que una chispa te dañe el ojo.
I will temper the metal, just in case you cool it too soon.
Yo templaré el metal, no sea que lo enfríes demasiado pronto.

no vaya a ser que

You will also frequently hear no vaya a ser que. This phrase means exactly the same thing as no sea que and is used interchangeably. Just like no sea que, it is always followed by the subjunctive.

Take the burning iron with the tongs, lest you burn yourself.
Toma el hierro ardiente con la pinza, no vaya a ser que te quemes.
I will have to melt the metal, lest the mold break.
Tendré que derretir el metal, no vaya a ser que el molde se rompa.