Of connects two things, showing belonging or composition: 'the roof of the house'. Off means away from or removed: 'Take the pan off the heat.' The double F belongs to the word about separation.
The core difference
Of is a linking preposition. It joins a part to a whole ('the end of the film'), a quantity to a substance ('a cup of tea'), or a quality to its owner ('a man of few words'). It never carries a sense of movement.
Off is the opposite instinct: detachment, removal, disconnection. It works as a preposition ('off the road'), an adverb ('the lid came off'), and an adjective ('the milk's off', 'the meeting is off').
Using each one
- Of: the beginning of March; a glass of water; one of my colleagues; the smell of rain.
- Off: He fell off the ladder. Turn the lights off. She took Friday off. Ten per cent off.
The test
Try 'away from' or 'removed'. If either captures the meaning, you want off: 'take it away from the shelf' → 'take it off the shelf'.
Otherwise, ask whether the word is joining two nouns. That is nearly always of. A useful sound check too: off is pronounced with a clear F sound, while of is pronounced 'ov'. Reading aloud catches most errors.
The mistake that matters most
By far the commonest error involving of has nothing to do with off. It is writing could of, should of, would of, must of and might of in place of could have, should have, and so on.
This happens because the contractions could've and should've sound almost exactly like 'could of'. The correct written form is always have:
- Wrong: I could of told you. Right: I could have told you.
- Wrong: You should of asked. Right: You should have asked.
The test is expansion. If the sentence would still work with a full 'have' — 'I could have told you' — then have is what you write, no matter what it sounds like.
One more pair to watch
Off of — 'get off of the table' — is standard in American speech but is widely disliked in British writing and adds nothing. Drop the of: 'get off the table'.
