Translation guide
The number 18. In Japanese, numbers are expressed using native Japanese or Sino-Japanese readings depending on context. For counting most things, the Sino-Japanese reading is standard.
Expressing the number eighteen in isolation, such as in math, phone numbers, or abstract counting.
Standard Sino-Japanese reading for the number 18. Used in most contexts: counting, math, phone numbers, etc.
Eighteen plus two is twenty.
When counting objects, 十八 is often followed by a counter suffix (e.g., 個, 人, 枚). The reading of the number part remains じゅうはち.
I bought eighteen apples.
Stating someone's age as eighteen.
Standard way to say '18 years old'. Note the sound change: はち + さい becomes はっさい.
He is eighteen years old.
Referring to the 18th day of a month.
Standard reading for the 18th day of the month. No irregular sound changes.
The meeting is on the 18th.
Counting 18 items using the generic counter 〜つ.
The native Japanese counter 〜つ only goes up to 9, so for 18 you must use 十八 plus an appropriate counter or just 十八 if the counter is obvious. The generic counter 個 (こ) is common.
There are eighteen (of them).
When 十八 (じゅうはち) combines with certain counters starting with さ, た, は, or か rows, the final ち may change to っ. For example, 十八歳 (じゅうはっさい) and 十八回 (じゅうはっかい).