These "nouns of locality" set limits on a number, level, or rank. 以上 = "and above," 以下 = "and below," 以内 = "within," 以外 = "beyond." They come after the amount. In standard usage 以上 / 以下 / 以内 are inclusive of the boundary number 含本数 (十岁以下 = ten and under, 10 included); only 以外 (and 不满 / 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 (十岁以下, not 以下十岁), and 以上 / 以下 / 以内 include the boundary number (十岁以下 = ten and under); only 以外 excludes it. Picture a number line: 以上 points up, 以下 down, 以内 stays inside, 以外 goes past.
Watch out
- 以内 = within a range; 以外 = beyond / except — don't mix the inside/outside pair.