là

1. The Equal Sign (A = B)
Use là to connect a subject to a predicate noun, pronoun, or possessive phrase. It establishes identity or ownership.

2. No Conjugation
Because Vietnamese has no verb conjugation, là never changes form based on the subject (unlike 'am', 'is', or 'are' in English). To change the time, simply add tense markers like đã (was/were) or sẽ (will be) before it. However, if a time word or context is already present, these tense markers are often omitted entirely.

3. When NOT to use là

• With Adjectives and Feelings: Vietnamese adjectives work like verbs. They already contain the meaning of 'to be', so you do not need là for basic descriptions or physical states/feelings (e.g., say Cô ấy đẹp, not Cô ấy là đẹp; say Tôi đói, not Tôi là đói). To make the sentence flow more naturally, speakers often use thì instead of là before adjectives (e.g., Cái áo này thì đẹp). Note: While advanced speakers sometimes use là before adjectives for strong emphasis, it must be paired with an intensifier like rất or quá (e.g., rất là đẹp). Using là completely alone before an adjective is ungrammatical.
• With Continuous Actions: Do not use là as an auxiliary verb to say you 'are doing' something. English uses 'to be + -ing', but Vietnamese uses the aspect marker đang before the verb (e.g., say Tôi đang ăn, not Tôi là ăn).
• With Locations: To say you are at a place, use ở instead of là.
• With Age: Do not use là to state your age. Just use the number and tuổi (e.g., say Tôi 20 tuổi, not Tôi là 20 tuổi).

4. Negation and Questions
To say "is not", use không phải là as the standard negation. While "không là" does exist, it is generally reserved for fixed phrases (like không là gì cả) or poetic/literary contexts. To ask a yes/no question with "là", use the frame có phải ... không? (e.g., Bạn có phải là sinh viên không? - Are you a student?).

5. Other Common Uses
Beyond meaning 'to be', là is frequently used as the conjunction 'that' after verbs of thinking or speaking (e.g., nghĩ là - to think that). It can also translate to 'As a...' or 'Being a...' when placed at the beginning of a sentence or clause to state a role or capacity (e.g., Là một giáo viên, tôi... - As a teacher, I...). It is also used to mean 'meaning' or 'that is' (nghĩa là, tức là) for definitions, and as a condition/result marker meaning 'then' or 'whenever' (e.g., Cứ mưa là ngập - Whenever it rains, it floods). Additionally, là is used to introduce a list or clarify a preceding noun, meaning 'namely' or 'which is/are' (e.g., Tôi biết hai ngoại ngữ, là tiếng Anh và tiếng Pháp). In spoken Vietnamese, là is also extremely common as a hesitation or filler word (similar to 'like' or 'um' in English). You will also see its related form phải used in tag questions like phải không? (right?).