Tuần này, tôi không rảnh vì có nhiều việc ở văn phòng.

Tuần này means this week and sets the time frame first. Vietnamese often puts time expressions near the beginning of the sentence.

So:

• Tuần này, tôi không rảnh... = This week, I’m not free...

This is very natural in Vietnamese. You could also say it without the comma in casual writing, but the meaning stays the same.

Rảnh means free, available, or not busy.

So không rảnh literally means not free or not available.

In this sentence:

• tôi không rảnh = I’m not free / I’m busy

It is a very common everyday word when talking about your schedule.

Both are natural, but they are slightly different in feel:

• không rảnh = not free
• bận = busy

So these are both possible:

• Tôi không rảnh.
• Tôi bận.

In many situations, they mean almost the same thing.
Không rảnh can sound a bit like “I don’t have free time,” while bận directly states “I’m busy.”

Vì means because.

It introduces the reason:

• Tôi không rảnh vì có nhiều việc ở văn phòng.
• I’m not free because there is a lot of work at the office.

So the structure is:

• statement + vì
  • reason

Very common patterns are:

• ... vì ... = ... because ...
• Vì ... nên ... = Because ..., so ...

In your sentence, only vì is used, which is completely normal.

Here có means something like there is/there are or to have, depending on how you analyze it.

• có nhiều việc = there is a lot of work / to have a lot of work

In this sentence, it expresses the existence of work that is causing the problem.

So:

• vì có nhiều việc = because there is a lot of work / because I have a lot of work to do

Vietnamese often uses có in this very flexible way.

Both are possible.

The sentence as given:

• ... vì có nhiều việc ở văn phòng

does not repeat the subject because it is already understood from context. This is very common in Vietnamese.

If you say:

• ... vì tôi có nhiều việc ở văn phòng

that is also correct, but it sounds a little more explicit.

So Vietnamese often leaves out subjects when they are obvious.

Việc means work, task, matter, or thing to do, depending on context.

Here, nhiều việc means:

• a lot of work
• many things to do

It is a very useful word. For example:

• Tôi có việc. = I have something to do.
• Hôm nay nhiều việc quá. = There’s too much work today.

In this sentence, ở văn phòng most naturally means at the office and connects to nhiều việc:

• có nhiều việc ở văn phòng = there’s a lot of work at the office

So the idea is that the office is the place where the work is.

It usually does not mean simply “I am physically at the office right now.”
It means the office is where the workload is coming from.

Vietnamese nouns usually do not change form for singular or plural.

So:

• việc can mean work, a task, or tasks, depending on context.
• nhiều việc clearly means many tasks / a lot of work because nhiều already shows quantity.

This is normal in Vietnamese: number is usually understood through words like:

• một = one
• hai = two
• nhiều = many / much

Tôi is correct and neutral. It is a safe default for learners.

However, in real conversation, Vietnamese speakers often choose pronouns based on age, relationship, and social context. Depending on who you are speaking to, you might hear or use:

• mình
• em
• anh
• chị
• con
• cháu

So tôi is grammatically fine and polite, but not always the most natural choice in every real-life situation.

Yes. Vietnamese often drops information that is already clear.

For example, in casual conversation, people might say:

• Tuần này không rảnh, vì có nhiều việc ở văn phòng.
• Tuần này bận vì có nhiều việc ở văn phòng.
• Tuần này bận lắm, ở văn phòng nhiều việc quá.

All of these sound natural in the right context.
Vietnamese often becomes shorter and less explicit in everyday speech.

The comma is helpful, but not absolutely necessary in informal writing.

• Tuần này, tôi không rảnh...
• Tuần này tôi không rảnh...

Both are understandable.

The comma simply marks Tuần này as a time phrase at the beginning, like a short pause in speech. It makes the sentence a little clearer and more polished in writing.