theo

Theo is extremely common in both casual conversations and highly formal contexts (like legal or academic texts).

Key Usages & Distinctions:

• According to: Used to cite sources or opinions (e.g., theo tôi - in my opinion). You can also use the slightly more formal variant theo như, which primarily introduces clauses (e.g., theo như tôi biết - as far as I know). For simple noun phrases, dropping như is the standard choice (e.g., theo kế hoạch - according to plan).
• Along (Spatial/Directional): Used to indicate following a path or direction (e.g., đi theo đường này - go along this road). You can also use dọc theo for moving along a physical feature (e.g., đi dọc theo bờ biển - walk along the beach).
• Bring / Take along: Combined with verbs of carrying or sending to mean "along" or "with" (e.g., mang theo / đem theo - to bring along, or kèm theo - attached/included).
• By / Per (Rates/Measurements): Used to indicate how things are measured or sold (e.g., bán theo cân - sold by the kilo).
• Sorting / Categorizing: Used for arranging or dividing things (e.g., xếp theo thứ tự - arrange by order, chia theo nhóm - divide by groups).
• Compliance / Standards: Used for complying with rules or standards (e.g., tuân theo - to comply, làm đúng theo quy định - do exactly according to regulations).
• Depending on (Variables): While tùy means "depending on," it is frequently combined with theo as tùy theo. Theo itself can also express variables (e.g., thay đổi theo thời gian - changes over time).
• Pursuing / Following (Abstract): Used for abstract pursuits, such as following a religion, pursuing a career, or siding with a group (e.g., theo đạo Phật - to follow Buddhism, theo nghề y - to pursue medicine, or theo phe - to side with).
• Theo vs. Dựa vào: Theo is used for citation ("according to"), while dựa vào means "based on" or "rely on" and can be used for both physical and abstract foundations (e.g., dựa vào kinh nghiệm - based on experience, dựa vào luật - based on the law).

Note: While often translated as a preposition, theo originates from the verb "to follow" and can function as the sole main verb to mean physically following or romantically pursuing someone (e.g., Con chó theo tôi - The dog follows me, or Anh ấy đang theo cô ấy - He is pursuing her). It frequently participates in a serial verb construction like làm theo (do according to / follow instructions), nghe theo (to obey / take advice), or học theo / bắt chước theo (to imitate / learn from someone's example), and forms the highly common phrase theo kịp (to keep up with).