as an adjective, 'least' describes a noun as being of the smallest importance, degree, or size.
That's the least important thing right now.
The least error can cause a huge problem.
This is my least favorite song.
when using 'least' as an adjective, it often comes before the noun it modifies, especially when it's used in a superlative sense.
as a pronoun, 'least' refers to the smallest amount or degree. It often stands in for a previously mentioned or understood noun.
Of all the students, he studied the least.
She eats the least of anyone I know.
I want the least of what's available.
'Least' as a pronoun can often be replaced by 'the smallest amount' or 'the fewest.'
as an adverb, 'least' modifies verbs, adjectives, or other adverbs to indicate the minimum degree or extent.
This is the least known fact about him.
She is the least interested in politics.
The least expected event occurred.
be cautious not to confuse 'least' with 'less.' 'Least' is the superlative form, indicating the absolute minimum, while 'less' is comparative.