This is the 659th most frequent Polish word.
zaczął
"Zaczął" means "he started" or "he began."
The verb 'zaczął' is used here to indicate the initiation of an activity or process by a masculine subject in the past tense.
On zaczął nowy projekt.
He started a new project.
The verb 'zaczął' here signals the beginning of a natural phenomenon, referring to rain starting.
Zaczął padać deszcz.
It started to rain.
In this sentence, 'zaczął' refers to beginning an action by a feminine subject in the past tense.