gì

Vietnamese is an isolating language, so gì never changes its form. Here is how you use it to ask "what":

• After a verb: ăn gì (eat what), làm gì (do what).
• After a noun: sách gì (what kind of book), món gì (what dish). Because Vietnamese has modifiers follow the noun, gì comes after the noun it describes. Note that gì asks an open-ended question (what kind of book), whereas nào asks you to choose from a restricted set (which book).
• After the verb 'to be' (là): là gì (is what).

Because Vietnamese uses wh-in-situ, question words simply stay in their natural grammatical slot. When asking a question, you do not move gì to the front of the sentence. Just build a normal sentence and replace the unknown information with gì. Note that while bare gì is commonly used as a subject or topic in short phrases like Gì vậy? or Gì thế? (What is it?), it generally cannot be the subject of an action verb; you must pair it with a classifier or a general noun. While cái gì (what thing) is the most common, gì pairs with a classifier or a general noun to ask about specific categories, such as con gì (what animal), người gì (what kind of person), or việc gì (what matter).

While bare gì generally cannot start a longer question as the subject, it is extremely common as a standalone question (Gì?) to mean 'What?' when asking someone to repeat themselves, or as an exclamation of shock or disbelief (Gì?! or Cái gì?!). To be polite to elders, it is better to use Dạ? or add a politeness particle like ạ.

Beyond asking questions, gì is also used to mean 'anything', 'nothing', or 'something'. Here are the most common patterns:

• Everything / Anything: Use gì cũng. The rule is simply [Noun + gì] + cũng + [Verb/Adjective]. When gì is the subject, it naturally sits at the front, as in Cái gì cũng đắt (Everything is expensive). When it is the object, it gets fronted/topicalized before the subject, as in Gì tôi cũng ăn (I eat anything). A very common conversational phrase is gì cũng được (anything is fine / whatever).
• Nothing: Use không ... gì. For example, không có gì (there is nothing / you're welcome). With action verbs, use the pattern không + [verb] + gì, such as Tôi không biết gì (I don't know anything). For emphasis, you can use không ... gì cả (nothing at all) in the North, or không ... gì hết / không ... gì hết trơn in the South. You can also use the pattern có ... gì đâu to downplay or deny something (e.g., Có sao gì đâu - It's nothing / No problem).
• Something: Adding đó to gì creates the meaning 'something' (gì đó or cái gì đó), as seen in the ai đó / gì đó pattern. (To say 'someone', you must use the question word ai to form ai đó). In casual speech, you can also use an indefinite gì without đó in affirmative sentences, such as Ăn gì đi (Let's eat something).
• Anything in Yes/No Questions: In yes/no questions, gì acts as an indefinite pronoun meaning 'anything' or 'something'. For example, Bạn có muốn mua gì không? (Do you want to buy anything?).