phải

1. Basic Sentence Structure
To express obligation, put phải directly before the main verb or verb phrase: Subject + phải + Verb. Because Vietnamese verbs never change form, phải stays exactly the same regardless of who is speaking.

2. Adding Time Markers
While phải itself doesn't change, you can add time markers before it to clarify when the obligation happens:

• đã phải: had to (past)
• sẽ phải: will have to (future)

3. Phải vs. Cần vs. Nên
Learners often confuse these three words:

• Phải (must): A strong external obligation imposed by rules, laws, or circumstances.
• cần (need to): An internal necessity or personal requirement.
• nên (should): A recommendation or good idea, but ultimately optional.

4. Other Meanings of Phải
Be aware that phải is a multi-functional word. Depending on the context, it can also mean 'right' (as in the direction, opposite of left) or 'correct' as a standalone adjective (e.g., Anh nói phải - You are right), which is often seen in the tag question phải không?. This tag is placed at the very end of a statement to mean '..., right?' or '..., isn't it?' (e.g., Bạn là sinh viên, phải không?). It is also used to frame Yes/No questions with có phải ... không? (Is it true that...?). To answer these questions affirmatively, you can simply use the standalone word Phải ('Yes' or 'That's right'). Furthermore, phải is heavily used to express a strong deduction or assumption, just like 'must be' in English. In everyday spoken Vietnamese, this is most naturally expressed as chắc phải or chắc chắn phải (e.g., Trời mưa, đường chắc phải trơn lắm - It's raining, the road must be very slippery). Additionally, when placed after a verb (Verb + phải), it indicates an accident, mistake, or unfortunate mishap (e.g., vấp phải - to trip over). You may also see it paired with passive markers for forced obligations (e.g., bị bắt phải - to be forced to).

5. The Negation Trap
Negating phải can be tricky for beginners. Here is what you need to know:

• không phải + Verb: means 'don't have to' (lack of external obligation). This is a standard way to say you are not required to do something, though native speakers frequently default to không cần or không cần phải in everyday speech to express 'don't have to / no need to', even for external rules.
• không cần (phải): means 'don't need to' (lack of internal necessity).
• không được + Verb: means 'must not' (prohibition). Be careful not to confuse this with không + V + được, which means 'cannot' (inability).

6. Negating the Verb 'To Be'
To say 'is not' or 'am not', you use không phải là + Noun. The là is often dropped in speech (Tôi không phải sinh viên), but phải stays. You cannot simply say không là to negate the verb là (to be).