có

1. Possession (To Have)
To say you own or possess something, place có directly after the subject.
Structure: Subject + có + Noun
Example: Tôi có tiền. (I have money.)

2. Existence (There is / There are)
To say something exists, simply place có before the noun. You can optionally put a location at the beginning of the sentence.
Structure: (Location) + có + Noun
Example: Ở đây có tiền. (There is money here.)

3. Yes-No Questions & Affirmation
To ask a yes-no question about an action or state, wrap the main word with có ... không?. To answer, you can reply with Có for "Yes" or Không for "No". It is equally standard to repeat the main verb or adjective (e.g., Hiểu for "Yes", Không hiểu for "No"). You can also use không có as a standalone answer to deny possession, existence, or to emphatically deny an action.
Structure: Subject + (có) + [Verb/Adjective] + không?
Example: Bạn có hiểu không? (Do you understand?) -> Có. / Không. (or Hiểu. / Không hiểu.)

Important Notes on Questions:

• Optional "có": When asking about adjectives (and often verbs in casual speech), the word có is optional (e.g., Bạn mệt không? - Are you tired?). Note that if a question omits có, it is much more natural to answer by repeating the adjective (e.g., Mệt) rather than saying Có, as có strongly implies possession or action.
• No Double "có": When the main verb is "to have", avoid doubling the word có in standard speech. Just say Bạn có tiền không? (Do you have money?). Doubling it (Bạn có có tiền không?) is only used for heavy, skeptical emphasis in highly colloquial spoken Vietnamese.
• With Nouns: If you use a noun in this structure (Subject + có + Noun + không?), it asks about possession (e.g., "Do you have money?"). To ask an identity question like "Are you a student?", you must use có phải là ... không?.
• Past Actions: You can use this pattern for past actions by adding a past time word (e.g., Hôm qua bạn có đi làm không? - Did you go to work yesterday?). Do not confuse this with đã ... chưa?, which specifically asks "Have you done it yet?".
• Subject Omission: In spoken Vietnamese, subjects are frequently dropped if the context is clear. Questions like Có hiểu không? or Có tiền không? are completely standard.

4. Emphasis (Did / Do / Did Not)
Place có directly before a verb to emphasize that an action really happened or does happen. To emphatically deny an action, use không có before the verb, or wrap the verb in có ... đâu (e.g., Tôi có biết đâu! - I didn't know at all!).
Structure: Subject + có / không có + Verb
Example 1: Tôi có đi làm. (I DID go to work.)
Example 2: Tôi không có đi làm. (I DID NOT go to work.)

Important Distinctions:

• Not for "To Be": Do not confuse có with To Be. Là links two nouns (A = B), while có shows existence or possession.
• Not for Location of Specific Items: Do not confuse có with At / In / On. Use ở when a specific, known thing is located somewhere (The book is on the table). Use có to introduce the general existence of something (There is a book on the table).