Tôi mua một phong bì.

I buy an envelope.

Breakdown of Tôi mua một phong bì.

• tôi: I
• một: a
• mua: to buy
• phong bì: the envelope

Questions & Answers about Tôi mua một phong bì.

What does each word in Tôi mua một phong bì do?

A simple breakdown is:

• Tôi = I
• mua = buy
• một = one / a / an
• phong bì = envelope

So the structure is:

• subject
  • verb
    • object

That makes it very straightforward for an English speaker.

Why is the word order so similar to English?

Because Vietnamese often uses Subject–Verb–Object word order, just like English.

So:

• Tôi = subject
• mua = verb
• một phong bì = object

That is why Tôi mua một phong bì lines up nicely with I buy an envelope.

Does một mean one or a/an here?

It can mean either, depending on context.

In this sentence, một most naturally works like a/an:

• Tôi mua một phong bì = I buy / bought an envelope

But it can also emphasize the number one if the context is about quantity.

So một often does double duty in Vietnamese:

• a number: one
• an indefinite article-like word: a/an