I
verb
BAD : At school I used to lack in confidence.
GOOD : At school I used to lack confidence.
BAD : She certainly doesn't lack of enthusiasm.
GOOD : She certainly doesn't lack enthusiasm.
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Do not confuse lack sth (verb + object, WITHOUT in/of ) with be lacking in sth and (a) lack of sth : 'He lacks the confidence to start his own business.' 'At school he was found to be lacking in confidence.' 'She certainly doesn't show any lack of confidence.' 'I think I'm suffering from a lack of sleep.'
II
noun
BAD : The room was fine except for a total lack of radio and television.
GOOD : The room was fine except that there was no radio or television.
BAD : Someone noticed the lack of salad dressing.
GOOD : Someone noticed that there wasn't any salad dressing.
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Lack is mainly used with abstract nouns: 'a lack of support/sympathy/ freedom/sleep/energy'
Avoid using lack with concrete nouns: 'We don't have any envelopes.' 'We're short of envelopes.' 'There aren't enough envelopes.'