'Lest' is followed by 'should' and conveys a negative purpose (to prevent something). It does not take 'not'. Correct: 'Lest you should fail'.

'Deny' means to say one did not do something. Adding 'not' makes it a double negative implying he did steal it. Correct: 'He denied that he had stolen the watch'.

Wait, this sentence is correct. Let's find an error: 'He admitted to have done it.' -> 'admitted to having done it'. Or 'He admitted that he is at fault.' -> Error 'is' (sequence of tense). Let's use: 'He denied that he had not stolen the money.' Error: 'not'. Explanation: Deny is negative.

'Unique' is absolute. Correct: 'This is a unique piece'.

Double comparative. Correct: 'stronger'.

'Little' is used for uncountable nouns. For countable nouns like 'friends', use 'few'. Correct: 'I have few friends'.

'Little' means hardly any (negative). 'A little' means some (positive). The proverb is 'A little knowledge is a dangerous thing'.

For family members, 'eldest' is preferred over 'oldest'. Correct: 'eldest member'.

'Did' is followed by the base form of the verb. Correct: 'did not go'.

'Since' indicates a period starting in the past continuing to now, requiring Present Perfect. Correct: 'I have been here since Monday'.