These "nouns of locality" set limits on a number, level, or rank. that level or higher以上 = "and above," that level or lower以下 = "and below," within以内 = "within," apart from以外 = "beyond." They come after the amount. In standard usage that level or higher以上 / that level or lower以下 / within以内 are inclusive of the boundary number 含本数 (十岁that level or lower以下 = ten and under, 10 included); only apart from以外 (and resentful不满 / to surpass超过) excludes it.
Structure: Amount / Level + 以上 / 以下 / 以内 / 以外
eighteen十八岁and over以上only then才may能enter进。
Only those eighteen and over may enter.
please请within在three days三天以内reply回复。
Please reply within three days.
Remember: the limit word comes AFTER the amount (十岁that level or lower以下, not that level or lower以下十岁), and that level or higher以上 / that level or lower以下 / within以内 include the boundary number (十岁that level or lower以下 = ten and under); only apart from以外 excludes it. Picture a number line: that level or higher以上 points up, that level or lower以下 down, within以内 stays inside, apart from以外 goes past.
Watch out
- within以内 = within a range; apart from以外 = beyond / except — don't mix the inside/outside pair.