Lan tưởng cậu đang bận.

The structure is:

• Lan = subject
• tưởng = thought / assumed / was under the impression
• cậu = you
• đang bận = being busy / busy at the moment

So the pattern is basically:

Subject + tưởng + clause

Vietnamese often puts ideas in a very direct order, so this sentence is literally something like:

Lan thought you were busy.

Tưởng often means more than a neutral think.

It commonly suggests:

• assumed
• thought mistakenly
• was under the impression

So Lan tưởng cậu đang bận can imply that Lan believed you were busy, and that belief may have been wrong.

This is different from nghĩ, which is a more general verb for think.

A rough comparison:

• Lan nghĩ cậu đang bận = Lan thinks / thought you were busy
• Lan tưởng cậu đang bận = Lan thought / assumed you were busy, often with a sense that it turned out not to be true

Yes, here cậu means you.

Vietnamese pronouns depend heavily on relationship, age, and social context. Cậu is a familiar pronoun, often used:

• between friends
• to someone of similar age
• in casual speech

So in this sentence, cậu is not a fixed word for you in every situation. It is just the pronoun that fits this relationship.

In another context, you could be:

• bạn
• anh
• chị
• em
• ông
• bà

and so on.

Đang marks an action or state as ongoing, similar to be ...-ing in English.

So:

• bận = busy
• đang bận = being busy / busy at that time

In this sentence, đang helps express the idea that the busyness was happening at that moment or during that period.

Without đang, the sentence would still be understandable:

• Lan tưởng cậu bận

That also means Lan thought you were busy, but đang makes the situation feel more immediate or in progress.

Bận is generally treated as an adjective meaning busy.

Vietnamese adjectives can function directly as predicates, so you do not need a word equivalent to English to be.

That is why:

• Cậu bận = You are busy

not something like Cậu là bận, which would be wrong.

With đang, you get:

• Cậu đang bận = You are being busy / You’re busy right now

Even though English uses to be, Vietnamese does not need it here.

Vietnamese does not have to mark tense the same way English does.

In English, you need was in Lan thought you were busy. In Vietnamese, time is often understood from:

• context
• time words
• aspect markers like đang
• the meaning of the main verb

Here, tưởng already places the belief in a particular viewpoint, and English often translates that with past tense. Vietnamese does not need a separate word for was.

So even without an explicit past marker, the sentence can naturally correspond to English thought you were busy.

Not always definitely, but it often does in practice.

Tưởng frequently describes a belief someone had before learning something else, so it often feels past-oriented:

• Lan thought you were busy.
• But actually, you were free.

However, Vietnamese tense is context-based. Depending on the situation, it could also mean something like:

• Lan is under the impression that you’re busy.

If the speaker wants to make the timing clearer, they can add time expressions such as:

• hôm qua = yesterday
• lúc nãy = earlier
• bây giờ = now

Yes. Vietnamese pronouns are flexible and depend on who is speaking to whom.

Cậu can mean:

• you, when addressing someone
• sometimes I/me, in certain pairings of pronouns
• in other contexts, it can even refer to a family role, such as maternal uncle

But in Lan tưởng cậu đang bận, the most natural reading is you.

This is one reason Vietnamese pronouns can feel tricky to English speakers: the same word does not always map to just one English pronoun.

Yes, there is a nuance difference.

• nghĩ = think
• tưởng = think/assume, often mistakenly

So:

• Lan nghĩ cậu đang bận = Lan thinks/thought you were busy
• Lan tưởng cậu đang bận = Lan thought/assumed you were busy, often with a hint that Lan was mistaken

If you want a more neutral statement, nghĩ is safer. If you want the idea of a wrong assumption, tưởng is better.

There are two main possibilities, depending on what you want to negate.

1. Negate the embedded idea:

• Lan tưởng cậu không bận.
• Lan thought you were not busy.

2. Negate tưởng itself:

• Lan không tưởng cậu đang bận.

This is grammatical, but it is less natural in many situations. It means:

• Lan did not think/assume you were busy.

Usually, Vietnamese speakers more often negate the part that contains the actual content of the thought, if that is what they mean.

Yes, it is very natural and colloquial.

It sounds like everyday spoken Vietnamese, especially because of:

• tưởng for assumed/thought
• cậu as a casual pronoun
• đang for an ongoing situation

A more formal or different version would mainly change the pronoun:

• Lan tưởng bạn đang bận.
• Lan tưởng anh đang bận.
• Lan tưởng chị đang bận.

The grammar stays the same; only the social relationship changes.