Tôi tưởng bưu điện đã đóng cửa.

Here, tưởng means something like thought, assumed, or was under the impression.

It often suggests that the speaker believed something, and that belief may have turned out to be wrong. So Tôi tưởng... is very commonly used when you say what you thought was true.

For example, this pattern is very common:

• Tôi tưởng anh không đi. = I thought you weren’t going.
• Tôi tưởng hôm nay nghỉ. = I thought today was a day off.

So in your sentence, tưởng is not just neutral thinking; it often carries the idea of a mistaken assumption.

Đã is a past or completed-action marker. In this sentence, it shows that the speaker thought the closing had already happened.

So bưu điện đã đóng cửa means the post office had already closed or was already closed.

This is different from simply saying:

• bưu điện đóng cửa = the post office closes / is closing / closes its doors

With đã, the sentence points more clearly to a completed event in the past from the speaker’s point of view.

It is literally made up of:

• đóng = close, shut
• cửa = door

But together, đóng cửa very often means to close in the sense of a shop, office, or business not being open.

So when talking about a place like bưu điện, đóng cửa is naturally understood as to be closed / to have closed for business, not just physically shutting one door.

Examples:

• Cửa hàng đã đóng cửa. = The shop has closed / is closed.
• Ngân hàng mấy giờ đóng cửa? = What time does the bank close?

Vietnamese does not use articles like a or the the way English does. So bưu điện can mean the post office or a post office, depending on context.

In this sentence, English usually translates it as the post office because the context probably makes it specific.

Also, classifiers are not always needed. A classifier would usually be used when counting or specifying a noun more explicitly, but here bưu điện is just the subject of the sentence, so no classifier is necessary.

Usually, bưu điện means post office.

In everyday Vietnamese, it normally refers to the physical place where you send mail, parcels, and so on. In some contexts, it can relate more broadly to postal services, but in a sentence like this, the most natural reading is the building or office itself.

So learners can safely understand bưu điện here as post office.

Vietnamese word order is often quite straightforward:

• Tôi = I
• tưởng = thought
• bưu điện = post office
• đã đóng cửa = had closed / was already closed

So the sentence is basically:

I thought + the post office + had closed

Vietnamese generally does not need extra words like that here, though you could think of the structure in English as:

I thought (that) the post office had closed.

This is a very normal Vietnamese sentence pattern:
subject + nghĩ/tưởng + clause

Very often, yes.

Tưởng commonly suggests that the speaker believed something but then discovered otherwise. So this sentence often implies something like:

• I thought the post office had closed, but actually it hadn’t, or
• I thought it was already closed, but apparently it was still open

That implication is one reason tưởng is so useful in conversation. It often carries a sense of surprise or correction.

However, context matters. In some cases, it can simply report what the speaker believed at the time, even if the sentence does not explicitly state whether it was wrong.

Yes, it could be left out, but the nuance changes.

• Tôi tưởng bưu điện đã đóng cửa.
  = I thought the post office had already closed.

• Tôi tưởng bưu điện đóng cửa.
  = I thought the post office closes / was closing / would close.

Without đã, the sentence is less clearly about a completed action. It becomes more general or more dependent on context.

So in your sentence, đã helps make the meaning more precise: the speaker thought the closing had already happened.

Sometimes yes, but đã and rồi are not always exactly the same.

You might hear:

• Tôi tưởng bưu điện đóng cửa rồi.

This also means something like I thought the post office had closed already.

Very roughly:

• đã often marks past/completion in a more grammatical way
• rồi often emphasizes that something has happened already

In many everyday contexts, both can work, but the feel is slightly different.
đã đóng cửa sounds a bit more neutral and structural.
đóng cửa rồi can feel a bit more conversational and can stress the already idea.

In Vietnamese, the line between verbs and adjective-like predicates is often less rigid than in English.

In this sentence, đóng cửa functions as the predicate describing what happened to the post office: it closed / was closed.

English forces you to choose between forms like:

• had closed
• was closed

Vietnamese does not make that distinction in exactly the same way. Bưu điện đã đóng cửa can cover the idea that the post office closed and the resulting state that it was closed.

So it is best to think of đóng cửa here as a predicate meaning to be closed / to have closed for business, depending on context.

It is very natural.

Tôi tưởng... is a very common way to say I thought..., especially when you later realize your assumption may have been wrong.
Bưu điện đã đóng cửa is also a normal and idiomatic way to talk about a post office being closed.

So the whole sentence sounds natural in everyday speech and writing. It is exactly the kind of sentence you might say if you arrived somewhere and were surprised it was still open.