I
verb
1
BAD : You should see a doctor or specialist to find out which type of diet will fit you.
GOOD : You should see a doctor or specialist to find out which type of diet will suit you.
BAD : A school like Summerhill will not fit for everybody.
GOOD : A school like Summerhill will not suit everybody.
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fit = (of clothes, shoes, rings, etc) be the correct size and shape for you: 'These trousers don't fit me any more.' 'The next size up should fit.'
suit = be suitable: 'Try and choose a career that suits you.' 'You should buy a dictionary that suits your needs, not just any one.'
2
BAD : Of course you will need some old paintings to fit the furniture.
GOOD : Of course you will need some old paintings to match the furniture.
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match (or go with ) = (of clothes, soft furnishings, etc) look good when seen together: 'We chose a dark green carpet to go with our yellow curtains.' 'I can't wear blue shoes with a black skirt - they don't match.'
II
adjective
BAD : A little exercise will help you to keep in fit.
GOOD : A little exercise will help you to keep fit.
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keep fit (WITHOUT in ): 'She keeps fit by jogging five miles every day.'
Compare: 'Cycling to work helps me to keep in shape.'