You already know that perché translates to "because" when giving a reason for an action. However, it can also mean "so that" or "in order to" when expressing a purpose or goal.
The meaning of perché changes completely depending on which verb mood follows it.
Stating a reason (Indicative)
When you use perché to state a factual reason or cause ("because"), use the indicative mood.
| I activate the burglar alarm because it prevents thefts. | Attivo l'antifurto perché previene i furti. |
| I bought an armored door because there are many intruders. | Ho comprato una porta blindata perché ci sono molti intrusi. |
Stating a purpose (Subjunctive)
When you use perché to express a goal or intent ("so that"), use the subjunctive mood. Because a goal is something you hope to achieve rather than an established fact, it requires the subjunctive.
Compare the first burglar alarm example above with the one below. The only difference is the verb mood (previene vs. prevenga), but the meaning shifts from a factual cause to an intended purpose.
| I activate the burglar alarm so that it prevents thefts. | Attivo l'antifurto perché prevenga i furti. |
Here are a few more examples of perché taking the subjunctive to express purpose:
| I close the armored door so that no intruder enters. | Chiudo la porta blindata perché nessun intruso entri. |
| We hide the camera so that the intruder doesn't see it. | Nascondiamo la telecamera perché l'intruso non la veda. |
| He changes the combination so that no one unlocks the safe. | Lui cambia la combinazione perché nessuno sblocchi la cassaforte. |