Discuss and learn French: French vocabulary, French grammar, French culture etc.
I am not sure to be able to explain the difference and the using. There is no really difference.
"there were so many people" = I would say "Il y avait tellement de gens". I wouldn't say "il y avait tant de gens". The music of the sentence is not usual. But "il y avait tant de monde" is usual and "il y avait tellement de monde" is usual too.
if the sentence needs to be continued, then use "tellement" : il y avait tellement de monde que je n'ai pas pu entrer.
f you have a hesitation, use "tellement". The sound of the sentence is more usual.
that's very helpful. good to know