which of the following sentences is the most common in English?
"all of the tasks were performed well." "all the tasks were performed well."
This is a job for Ngrams.
Both sentences are grammatically correct and semantically equivalent, but all the tasks is more common.
You can find this sort of thing out by searching corpora like The Corpus of Contemporary American English (COCA) or The British National Corpus (BNC). I searched for both strings in both corpora. Here's what I found:
COCA BNC all of the tasks 8 1 all the tasks 42 12
So it seems that both phrases are used, but the shorter version is more common in both British and American English. Before regular noun phrases like the tasks, they seem to be roughly interchangeable.
This agrees with in Swan's Practical English Usage, point 36, in which he says either all or all of can be used. He notes, however, that before pronouns, all of is used. He gives the following examples:
All of us can come tomorrow.
She's invited all of you.
Note that the accusative form us is used, not the nominative we.
Both are okay but... GrammarMonster describes it.
All of the tasks were performed well - is not preferred. Because tasks is not a pronoun.
Very basic rule says that we generally use all of when the next word is a pronoun. For instance...
All of 'you', all of 'us'
For normal nouns, we generally use just all.
All the 'surgeons', all the 'soldiers', all the 'tasks' and so on.