1An adverb indicating that something applies to almost all or nearly the entire amount—'almost', 'nearly', 'mostly'.
今日の授業はほとんどオンラインで行われた。
Almost all of today's classes were held online.
彼の話はほとんど本当だったが、細かい部分に間違いがあった。
What he said was almost entirely true, but there were a few small mistakes in the details.
この箱の中にはほとんど何も入っていない。
There's almost nothing in this box.
クラスの学生はほとんど日本人だ。
Almost all of the students in the class are Japanese.
2Used together with a negative verb in the pattern 'ほとんど〜ない' to mean 'hardly', 'barely', or 'scarcely'.
昨日は忙しくて、寝る時間がほとんどなかった。
I was so busy yesterday that I hardly had time to sleep.
その薬は効き目が弱く、痛みはほとんど取れなかった。
That medicine was weak, and the pain was hardly relieved at all.
彼女は日本語がほとんど分からないと言っていた。
She said she barely understood any Japanese.
3Used as a noun meaning 'the large majority' or 'most (of)'; can take の to modify another noun.
参加者のほとんどが会社員だった。
The large majority of the participants were company employees.
その意見に、ほとんどが賛成した。
The large majority agreed with that opinion.