HSK 4Grammar
语气副词 — 底
Explanation
In HSK 4 grammar, under 词类 → 副词 → 语气副词, the forms include: 底、难道、千万、确实、只好、差（一）点儿.
Examples
底
dǐ
bottom; end
难道
nándào
could it be that
千万
qiānwàn
ten million; by all means
确实
quèshí
indeed; really
只好
zhǐhǎo
have to; only
差
chà
to differ; poor
一
yī
one
点儿