The least is used before uncountable nouns. It is the superlative of little.
- He does the least work in this office.
The least can be used without a noun if the meaning is clear from the context.
- Jane does the most work in this office. Jack does the least.
The expression ‘least of’ can be used before plural abstract nouns.
- ‘She will be really upset when she hears about this.’ ‘That’s the least of my worries.’
In questions and negative clauses, the least can be used with the meaning ‘any…at all’ before singular abstract nouns.
- ‘What’s the time?’ ‘I haven’t got the least idea.’
- I’m not the least bit afraid of dogs.
The fewest is the superlative form of few.
- The essay with the fewest grammatical mistakes isn’t always the best.
The least can be used before adjectives. It is the opposite of the most.
- The least expensive picnics are often the most enjoyable.
- The most expensive things aren’t always the best.
At least means ‘not less than’.
- She is at least forty years old. (= She is not less than but more than forty years old.)
- He interviewed at least four times this month.
- I have seen that film at least ten times.
- She has been in love at least six times.
Not in the least
Not in the least means ‘not at all’. It is used in a formal style.
- I was not in the least impressed by her manners. (= I was not at all impressed by her manners.)