Stare has a second trick. Stare per + infinitive means something is about to happen:

Italian	English
Sto per uscire.	I'm about to go out.
Il film sta per cominciare.	The film is about to start.
Sta per piovere.	It's about to rain.

Compare the two:

• Sto uscendo. — I'm going out (I'm on my way out the door).
• Sto per uscire. — I'm about to go out (any minute now).

Stare per takes the infinitive — uscire, not uscendo.