verb
1
BAD : My shoes were filled with water so I took them off.
GOOD : My shoes were full of water so I took them off.
BAD : The eight o'clock train is usually filled with office workers.
GOOD : The eight o'clock train is usually full of office workers.
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be filled with = containing so many or so much that there is no empty space left: 'The first drawer was filled with neat piles of shirts.' 'The front page is filled with the most important news items.'
be full of = (1) containing a large number or amount: 'The kitchen was full of flies.' 'The essay was full of mistakes.' (2) be filled with: 'The kettle was full of boiling water.'
2
BAD : You'll have to fill an application form.
GOOD : You'll have to fill in an application form.
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fill in/out a form, coupon etc: 'To open a new account, you have to fill in an application form.' 'Remember to fill out the counterfoil.'
fill in an empty space on a form, coupon etc: 'Just fill in Section A.'
fill in details on a form: 'Fill in your name and address and then sign the form at the bottom.'
Note that fill out is more common in American English.