mấy

1. Asking 'How Many' (Small Numbers)
When you expect a small number (usually under 10), use the formula mấy + classifier + noun. For example: mấy quyển sách (how many books). If the word acts as its own measure word or classifier, mấy goes directly before it, such as asking a child their age: cháu mấy tuổi? If the context is clear, the noun or classifier can be dropped entirely, allowing mấy to act as a standalone pronoun (e.g., Bạn lấy mấy? = How many do you want to take?).

2. Duration vs. Sequence
The position of mấy changes the meaning between a quantity and a specific point in a sequence. Placing mấy before the noun asks for a quantity or duration (e.g., mấy ngày = how many days). Placing it after the noun asks for a specific point in a sequence (e.g., thứ mấy = what day of the week, ngày mấy = what date, tháng mấy = what month). Note that for the first 10 days of the month (and especially for the Lunar calendar or Tết), Vietnamese people often prefer to use mùng mấy as an alternative to ngày mấy. Exceptions: Asking for the time is a notable exception where mấy goes before the noun: mấy giờ (what time). Note that mấy giờ can technically also mean 'how many hours' (duration), so to avoid ambiguity, Vietnamese speakers usually use mấy tiếng or mấy giờ đồng hồ for duration. Also, while năm nào is often used to ask 'what calendar year', năm mấy is extremely common in everyday speech (especially in the South) for calendar years as well (Năm nay là năm mấy?), along with birth years (Bạn sinh năm mấy?) or school years (Bạn học năm mấy?).

3. Meaning 'A Few' in Statements
If you use mấy in a regular sentence (not a question), it simply means 'a few' or 'some'. For example: Tôi có mấy quyển sách (I have a few books).

4. Mấy vs. Bao Nhiêu
If you expect a large number (over 10) or are asking about uncountable things like money or prices, you generally use bao nhiêu instead of mấy. However, note that bao nhiêu can also be used for small, countable numbers; it just sounds slightly more formal or objective. Mấy can be used for large numbers if combined with multipliers like chục (tens) or trăm (hundreds), such as mấy chục người (several tens of people) or mấy trăm ngàn (several hundred thousand). Additionally, mấy can be placed after a multiple of ten to act as a placeholder for a single digit (1-9), meaning '-something'. For example, mười mấy means 'ten-something' (11-19), and hai mươi mấy means 'twenty-something' (21-29).

Note: The 'under 10' rule is a general guideline based on the speaker's expectation, not a strict mathematical limit. If you expect a small amount, use mấy even if the actual answer turns out to be 12!

5. Meaning 'Not Very' (Không ... Mấy)
When combined with không and an adjective or verb, mấy means 'not very' or 'not much'. For example: không ngon mấy (not very delicious) or không thích mấy (don't like it much). A slightly more formal alternative is to place mấy before the adjective or verb: không mấy vui vẻ (not very happy). Additionally, the highly common fixed phrase không mấy khi means 'rarely' or 'seldom'.

6. Casual Plural Marker for People
In everyday speech (especially in Southern Vietnamese), mấy acts as a casual pluralizing word for people. You will often hear it used with nouns like mấy đứa (those kids / you guys) or mấy người (those people), and combined with pronouns such as mấy ổng (those guys) and mấy bả (those ladies).

7. Exclamations and Rhetorical Questions
Mấy is also used in expressive patterns. The phrase biết mấy means 'so much' or 'extremely' in exclamations (e.g., Vui biết mấy! = So incredibly happy!). Additionally, the rhetorical pattern mấy khi means 'rarely' or 'seldom' (e.g., Mấy khi được gặp nhau = It's rare that we get to meet).