Discuss and learn French: French vocabulary, French grammar, French culture etc.
I think the difference is very very very little.
I think subitement means more "fast" (and unexpected) and soudainement means more "unexpected" (and fast)
But it's not really obvious.
Yes the both are commonly used. Some words are used with one or the other. "il est mort subitement" . we don't say "il est mort soudainement".
i first learned "tout a coup" so i was surprised when i encountered recently the other two.
the italian word "subito" means "immediately." maybe that sheds some light
i would also add that in english, i think "suddenly" connotes more that something arose unexpectedly, "out of the blue"
"i suddenly realized"... versus "i immediately realized" or "i was walking down the street when all of a sudden i heard a loud noise." the emphasis is more on the unexpected occurrence than the immediacy i would say.