verb
1
BAD : The criminal mind is not always easy to realize.
GOOD : The criminal mind is not always easy to understand.
BAD : I didn't realize the meaning of 'junk mail'.
GOOD : I didn't understand the meaning of 'junk mail'.
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KNOW · REALIZE · RECOGNIZE · UNDERSTAND
Know be aware of a fact or particular piece of information: ‘Do you know their new telephone number?’ ‘I know she’s fond of you. She told me so herself.’
Realize suddenly become aware of a fact or the true meaning of something, especially because you have though about it or received new information about it: ‘I suddenly realized that the thumping I could hear was the sound of my own heart.’ ‘It was only when I saw the expression on the doctor’s face that I realized the seriousness of her illness.’
Recognize know what something is because you have seen/heard/smelled it before; know who someone is because you have seen them before: ‘I’m sorry I didn’t recognize you - you’ve had your hair cut!’
Understand know what something means, what causes something, how something operates, how someone feels etc: ‘I’ve never really understood physics.’ ‘Such behaviour is very difficult to understand.’
2
BAD : The school year is divided into three terms, as you realize.
GOOD : The school year is divided into three terms, as you know.
BAD : Nobody realized whether he was coming.
GOOD : Nobody knew whether he was coming.
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See Language Note above
3
BAD : Our own mistakes are sometimes difficult to realize.
GOOD : Our own mistakes are sometimes difficult to recognize.
BAD : I'm sure you will realize the song when you hear it.
GOOD : I'm sure you will recognize the song when you hear it.
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See Language Note above
4
BAD : The headmaster realized his threat and sent the children home.
GOOD : The headmaster carried out his threat and sent the children home.
BAD : Many people in developing countries do not realize family planning.
GOOD : Many people in developing countries do not practise family planning.
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When realize means 'make something actually happen or become real', it is usually used with words such as ambition, intention, expectation, hope, dream, fear : 'I feel confident that the high expectations I have of my new job will be realized.'