verb
BAD : On the plane home, I suddenly recognized that I had left my coat at the hotel.
GOOD : On the plane home, I suddenly realized that I had left my coat at the hotel.
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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.’