El fuego es peligroso; nunca lo dejes sin supervisión.

A semicolon in Spanish (just like in English) links two closely related independent clauses without the full stop of a period, but with a stronger separation than a comma. Here:

• “El fuego es peligroso” is a complete thought.
• “Nunca lo dejes sin supervisión” is another.
  Using a semicolon emphasizes their connection (cause and effect) without breaking the flow too abruptly.

The pronoun lo is a masculine singular direct‐object pronoun that replaces el fuego. Instead of saying “nunca dejes el fuego”, the sentence uses “lo” to avoid repetition:

• “El fuego” → “lo”

In Spanish you can form negative commands either by:

1. Placing no before the verb: No lo dejes sin supervisión.
2. Using a negative adverb like nunca or jamás at the front: Nunca lo dejes sin supervisión.
   Both are correct. Here the speaker chose nunca to mean “never,” which already conveys the negative, so no is omitted.

Spanish negative tú commands use the present‐subjunctive. For the verb dejar:

• Affirmative tú imperative: deja
• Negative tú imperative: no dejes or nunca dejes
  Since this is a negative command (“never leave it”), dejes is the correct subjunctive form.

After sin, Spanish generally drops the definite article if you mean supervision in an abstract or general sense:

• Correct: sin supervisión (“without supervision” in general)
• With la supervisión would sound like you refer to a specific supervision you’ve already mentioned.

Yes. Some natural alternatives:

• No dejes el fuego desatendido.
• Jamás lo dejes sin atención.
• Nunca dejes el fuego sin vigilancia.
  All convey the same “never leave the fire unsupervised” warning.