A casa riscaldo la zuppa con cura per non farla bollire troppo.

The present tense in Italian can express:

1. An action happening right now (“I’m heating the soup”).
2. A habitual action (“Whenever I’m at home, I heat the soup”).
   Using riscaldo here could be either “I’m reheating the soup” or “when I get home, I reheate it (as usual).”

Both mean “to heat,” but with a nuance:

• scaldare = heat (something) from a cold state; can also mean “to scald” (burn with hot liquid).
• riscaldare = heat again or keep warm (reheat).
  Since the soup was already cooked and cooled, you use riscaldare.

1. per
   • infinitive expresses purpose (“in order to…”).
2. non negates the infinitive that follows.
3. far is the causative verb (“to make/let [something] do something”).
4. la = direct object pronoun referring back to la zuppa.

Putting it together:

• per non = “so as not to”
• farla = “make it” (far + la)
• bollire = “boil” (infinitive)
• troppo = “too much”

So per non farla bollire troppo = “so as not to make it boil too much.”

Yes. Both are grammatically correct and convey almost the same idea. Differences:

• non farla bollire troppo uses a pronoun, is a bit more compact.
• non far bollire troppo la zuppa repeats the noun, maybe clearer for beginners.

Yes. A common alternative is affinché + subjunctive. For example:

• Affinché la zuppa non bolla troppo, la riscaldo con cura.
  That’s more formal and requires the subjunctive (non bolla rather than non bolli here if it were true subjunctive, it’d be non bolla is indicative; the true subjunctive would be non bolla, but typically you’d see non bolla replaced by non bolla only in fixed expressions). In practice, per
  • infinitive is simpler and more everyday.