In Vietnamese, tại sao is used to express “why”.
Vietnamese is a wh-in-situ language, which means, in Vietnamese, the question words aren’t moved to the beginning of sentences, but rather stay in their original positions.
Structure
Place tại sao at the beginning of the sentence.
Tại sao + clause?
Examples
- Tại sao bạn học tiếng Việt?Why do you learn Vietnamese?
- Tại sao anh không đến?Why don’t you come?
- Tại sao là tôi?Why me?
Alternative form
Another phrase to express “why” is vì sao. The difference is subtle; vì sao sounds more friendly and carries a sense of “genuinely wondering” while tại sao sounds more neutral and more formal. Overall, tại sao is the more common phrase (by quite a margin).
- Vì sao châu Âu giàu có?Why is Europe rich?
Trivial
Vì sao also means “star”. It is a [measure word + noun] structure, in which sao, the noun, means “star”, and vì acts as the measure word. Note that vì sounds quite poetic, so it mostly appears in poems, songs, etc. The casual and more common measure word for sao is ngôi.