Duration, Frequency, and Age

Vietnamese uses numbers with time units to answer several related questions: bao lâu “how long?”, bao lâu một lần “how often?”, mấy lần “how many times?”, and bao nhiêu tuổi / mấy tuổi “how old?” The surface forms look simple, but word order and framing change whether hai giờ identifies a point at two o'clock or measures two hours. Vietnamese often leaves English for, per, and old untranslated because the unit expression and sentence structure already make the relation clear.

This page covers elapsed duration, repeated frequency, and age. It does not reteach basic zero-to-ten forms, which belong on Cardinal Numbers from Zero to Ten.

A decision aid

These unit nouns do not require a general classifier. Compare ba cái đồng hồ “three clocks,” where cái counts objects, with ba giờ “three o'clock/three hours,” ba lần “three times,” and ba tuổi “three years old,” where the following word itself supplies the unit.

Measuring duration

A bare duration commonly follows the verb phrase:

For elapsed hours, tiếng is often especially natural: hai tiếng “two hours.” Giờ also functions as an hour unit, particularly in measurements and formal statements, but it can be confused with a clock point. Lúc hai giờ unmistakably means “at two o'clock”; làm hai tiếng means “work for two hours.”

The position of rưỡi remains unit-final: một tiếng rưỡi, hai ngày rưỡi, ba tháng rưỡi. With a calendar span, trong can present a bounded interval or time available: trong ba ngày “within/over three days.” Do not mechanically insert it for every English for.

Ongoing duration and endpoints

For a situation that began earlier and still holds, a common frame is đã + verb + được + duration + rồi. Được measures what has accumulated; rồi anchors the amount as reached by now.

Endpoints use từ... đến... “from... to...” or từ... tới.... Trong vòng sets an outer limit and is common in instructions and official writing.

English distinguishes for from since overtly. Vietnamese often lets từ introduce the starting point—từ năm 2020—while an accumulated duration takes được... rồi.

Expressing frequency

Lần counts occurrences: một lần “once,” hai lần “twice,” and ba lần “three times.” Add a cycle phrase to say “per.”

The questions select different measurements. Bao lâu một lần? asks for the interval between recurrences, whereas Mỗi tuần mấy lần? asks for the number of occurrences within a named cycle. Bare Mấy lần? may simply ask for a total count, so it is not by itself an exact equivalent of English how often?

Both hai lần một tuần and mỗi tuần hai lần are natural. The first packages a rate; the second chooses the cycle as topic. Mỗi distributes the event over individual cycles: mỗi ngày, mỗi tháng, mỗi năm. Hằng ngày, hằng tháng, hằng năm are neutral-to-formal habitual expressions and are especially common in writing.

For regular recurrence at an interval, use cứ + interval + lại + event or cứ + interval + event + một lần.

The two frames differ in discourse focus. Cứ... lại... highlights recurring return; ... một lần presents a rate. Neither requires an English preposition per.

Stating and asking age

Age uses tuổi after a cardinal: mười lăm tuổi, hai mươi mốt tuổi, ba mươi tư tuổi. Ask a child or peer Bao nhiêu tuổi? or Mấy tuổi? only when the relationship makes the question appropriate. With an older or respected addressee, include a correct kinship term and ạ, but remember that age itself can be sensitive.

Tuổi is an age unit, so there is no classifier between the number and tuổi. When counting people rather than their ages, however, use the appropriate human unit: hai mươi mốt người “twenty-one people.” Infants' ages are often counted in months: mười tám tháng tuổi “eighteen months old.”

Approximate age expressions include hơn ba mươi tuổi “over thirty,” gần bốn mươi tuổi “nearly forty,” and ngoài ba mươi “in one's thirties / over thirty,” whose exact interpretation depends on context. In official settings, state a birth year or exact age rather than relying on an approximation.

One number system, several readings

Quantities such as hai nghìn ngày, ba triệu phút, or một tỷ giây are grammatically possible because nghìn, triệu, and tỷ build the number while the final unit identifies what is measured. Natural discourse usually chooses a more manageable calendar unit.

A schedule with three interpretations

The paragraph uses the same number grammar for age, duration, frequency, clock time, and date, but the unit frame changes the interpretation each time. Hai tiếng measures length; mỗi ngày sets frequency; lúc tám giờ rưỡi locates an event.

Common Mistakes

1. Using lúc for a duration

Remove lúc and use an elapsed-time unit to measure duration.

2. Adding a classifier before a time or age unit

Vietnamese classifiers count ordinary objects; measure units generally stand directly after the numeral.

3. Missing positional number forms

The same naturalness principle gives ba mươi tư and mười lăm.

4. Translating per mechanically

Vietnamese can juxtapose the occurrence count and cycle without a preposition.

5. Asking age without ranking the participants

The repair uses an age-appropriate address term and ạ. Politeness cannot make every personal question appropriate, so context still matters.

Key Takeaways

• Let lúc locate a clock point; let a bare time-unit phrase measure duration.
• Use đã... được + duration + rồi for an amount accumulated up to now.
• Count occurrences with lần and cycles with mỗi, hằng, or cứ... lại....
• Put tuổi directly after the age number, with no object classifier.
• Keep number allomorphs and social address accurate: grammar and participant ranking both shape a natural sentence.