This is the 649th most frequent Greek word.
άρχισε
"Άρχισε" means "started" or "began" in English.
In this context, 'άρχισε' (started) expresses the onset of a weather condition.
Άρχισε να βρέχει ξαφνικά.
It suddenly started to rain.
The verb 'άρχισε' (started) indicates the beginning of an action, in this case, reading.
Ο Κώστας άρχισε να διαβάζει το βιβλίο του.
Kostas started reading his book.
Here 'άρχισε' (started) is used to specify the commencement time of an event.