"Begin" might also be for something that has already started. But to "start" marks the actual/exact time of launching an activity (to understand more clearly, consider these two examples: This is just the beginning [meaning, all the initial period] . It's 10:00 o'clock, folks; let's get started [whenever we talk about a specific time, we rather use the word "start""start"]. Since we are bound by the two sentences above, we cannot speak of the noun-verb possibility of each word as a difference.
Soulaimane (Mr. SOUL)