A number is only part of a useful expression. 三時 is three o’clock, 三時間 is three hours, and 三人 is three people. The attached word tells you what is being counted. Practise the whole expression rather than assuming a digit always has one reading.
Build prices from place values
Start with 一（いち）, 二（に）, 三（さん）, 四（よん）, 五（ご）, 六（ろく）, 七（なな）, 八（はち）, 九（きゅう） and 十（じゅう）. 十五 is 15; 二十 is 20; 二十五 is 25. At hundreds, notice 三百（さんびゃく）, 六百（ろっぴゃく） and 八百（はっぴゃく）. At thousands, 三千 is さんぜん and 八千 is はっせん.
2,350円 is 二千三百五十円（にせんさんびゃくごじゅうえん）. Japanese groups large numbers around 万, 10,000: 12,000 is 一万二千, not “twelve thousands” translated word by word. 円 follows the amount.
Clock times have a few fixed readings
| Written | Reading | Meaning |
|---|---|---|
| ４時 | よじ | four o’clock, not よんじ |
| ７時 | しちじ | seven o’clock |
| ９時 | くじ | nine o’clock |
| １分 / ３分 / ６分 | いっぷん / さんぷん / ろっぷん | one / three / six minutes |
| ８分 / １０分 | はっぷん / じゅっぷん | eight / ten minutes; じっぷん is also used |
| ２時半 | にじはん | half past two |
| ２時間半 | にじかんはん | two and a half hours |
午前 means a.m.; 午後 means p.m. 午後３時 is 3 p.m. In an appointment, repeat the whole date and time: 金曜日の午後３時ですね (Friday at 3 p.m., right?). A number heard correctly with the wrong unit still creates the wrong appointment.
Dates: learn the common irregular set
The first ten days of a month are ついたち, ふつか, みっか, よっか, いつか, むいか, なのか, ようか, ここのか and とおか. Also learn 14日（じゅうよっか）, 20日（はつか） and 24日（にじゅうよっか）. Most other dates combine a number with にち, but 17日 and 19日 use じゅうしちにち and じゅうくにち.
Month names include ４月（しがつ）, ７月（しちがつ） and ９月（くがつ）. The day of the month and a duration are different: ついたち is the first day, whereas いちにち can mean one day. Keep a calendar example on the same card as the reading.
Two counters you can use immediately
For people: 一人（ひとり）, 二人（ふたり）, 三人（さんにん）, 四人（よにん）. For a general count of things, learn ひとつ, ふたつ, みっつ, よっつ, いつつ, むっつ, ななつ, やっつ, ここのつ and とお. Specific objects may use a different counter, so this general set is a starting tool, not a replacement for every counter.
1. Read ４時半.
よじはん: half past four. 四時 has the fixed reading よじ.
2. Does ３時間 mean 3 p.m.?
No. さんじかん is a duration of three hours. 午後３時 is 3 p.m.
3. Read ９月２０日.
くがつはつか: September 20. Both the month and day have readings worth learning as whole expressions.
4. Ask for two coffees using the general counter.
コーヒーをふたつください. In a café this is a practical order: two coffees, please. The drink-counter form 二杯（にはい） is also possible.