We can use a Group B determiner directly before nouns. We don’t have to use of.
- We need to buy some sugar. (NOT We need to buy some of sugar.)
- Most people like to travel. (NOT Most of people like to travel.)
However, a Group B determiner cannot be used directly before a noun which has a Group A determiner (articles, possessives or demonstratives). In such cases we have to use of.
Compare:
- Most people / most of the people (NOT most the people)
- Each child / each of the children (NOT each the children)
- Neither answer / neither of the answers (NOT neither the answers)
- Five green apples / five of the green apples (NOT five the green apples)
Notes
No and every are not used before of; instead we use none and everyone.
Compare:
- He has no friends.
- None of his friends wished him on his birthday. (NOT No of his friends …)
- Every boy has handed in his work.
- Everyone of the boys has handed in his work. (NOT Every of the boys has …)
Of can be left out after all, both and half when they are followed by nouns.
- All of his ideas are interesting. OR All his ideas are interesting.
- Both of my parents live in India. OR Both my parents live in India.
Note that of cannot be left out when all, both and half are followed by pronouns.
- All of us like to travel. (NOT All us like to travel.)