adverb
Used to describe that something is close to a complete state or quantity, but not entirely. Common in both spoken and written Japanese.
宿題はほとんど終わった。
I've almost finished my homework.
Most of the customers at this shop are women.
adverb
Used with a negative verb to mean 'hardly' or 'barely'. Emphasizes that something is almost not the case.
彼はほとんど食べなかった。
He hardly ate anything.
その話はほとんど信じられない。
That story is barely believable.
noun, noun which may take the genitive case particle 'no'
most; the majority; almost all
Used as a noun or with の to refer to the greater part of something. Often interchangeable with sense 1, but explicitly nominal.
参加者のほとんどが賛成した。
Most of the participants agreed.
ほとんどの人がその事実を知らない。
Almost all people don't know that fact.