Lan đã chọn áo màu xanh cho bữa tiệc tối nay.

Đã marks the action as completed. Here, it tells you that Lan has already chosen or chose the outfit.

So:

• Lan chọn áo màu xanh... = Lan chooses / chose a blue shirt, depending on context
• Lan đã chọn áo màu xanh... = Lan has chosen / chose the blue shirt

A useful thing to remember: Vietnamese does not have tense in the same way English does. Words like đã, đang, and sẽ often help show time or aspect.

Vietnamese usually puts the noun first and the describing words after it.

So:

• áo = shirt/top
• màu xanh = blue color

Together, áo màu xanh means a blue shirt/top.

This is different from English, where the adjective usually comes first:

• English: blue shirt
• Vietnamese: shirt blue/color blue

You may also hear áo xanh, which is a more direct and very common way to say blue shirt.
Áo màu xanh is a bit more explicit, like a shirt that is blue in color.

It can mean either blue or green, depending on context.

In Vietnamese, xanh is a broad color word. To be more specific, people often say:

• xanh dương / xanh da trời = blue
• xanh lá cây = green

So màu xanh by itself can be ambiguous. In your sentence, the intended meaning is already known, so it is understood as blue.

Áo usually means a shirt, top, or upper-body garment.

It is a broad word, and its exact translation depends on context. For example:

• áo sơ mi = shirt
• áo thun = T-shirt
• áo khoác = jacket

In this sentence, áo màu xanh most naturally means a blue shirt/top.

Here, cho means for.

It shows the purpose of the choice:

• chọn áo màu xanh cho bữa tiệc tối nay
  = choose a blue shirt for tonight’s party

So cho links the shirt to the event it is meant for.

Be careful: cho can also mean to give in other sentences, so its meaning depends on structure and context.

Because it is a phrase giving extra information about what the shirt is for.

The core sentence is:

• Lan đã chọn áo màu xanh.
  = Lan has chosen a blue shirt.

Then Vietnamese adds:

• cho bữa tiệc tối nay
  = for tonight’s party

Putting this kind of purpose phrase after the main object is very normal in Vietnamese.

Both are possible, but bữa tiệc sounds very natural for a party/feast/banquet occasion.

• tiệc = party, banquet
• bữa tiệc = the party occasion / the feast / the event

The word bữa often refers to an occasion or meal event, so bữa tiệc feels a bit fuller and more natural in many situations.

So:

• cho bữa tiệc tối nay = for tonight’s party
• cho tiệc tối nay = also understandable, but can sound slightly less complete depending on context

Vietnamese does not use articles like English a, an, and the.

So áo màu xanh could mean:

• a blue shirt
• the blue shirt

The exact meaning depends on context.

Vietnamese often leaves this kind of information unstated unless it is important. Context usually makes it clear.

Because classifiers are not always required.

Vietnamese often uses classifiers when the noun is being counted or pointed out more specifically:

• một cái áo = a shirt
• cái áo đó = that shirt

But in a sentence like this, áo màu xanh is completely natural without a classifier.

If you added one, it would sound like:

• Lan đã chọn cái áo màu xanh cho bữa tiệc tối nay.

That can sound more like the blue shirt or a more specific item already identifiable in context.

Yes, it could.

Vietnamese often relies on context or time expressions rather than mandatory tense marking. For example:

• Lan chọn áo màu xanh cho bữa tiệc tối nay.

This could still be understood, especially if the context already makes the time clear.

However, đã makes it clearer that the choosing is completed. It tells the listener that Lan has already made the choice.

That meaning comes from tối nay.

• tối = evening/night
• nay = this / now, in time expressions

So tối nay means tonight.

In bữa tiệc tối nay, it modifies bữa tiệc, giving:

• the party tonight
• more naturally in English: tonight’s party

This is another example of Vietnamese putting descriptive information after the noun.

Yes. The sentence follows a very common Vietnamese pattern:

Subject + aspect marker + verb + object + extra phrase

Breaking it down:

• Lan = subject
• đã = completed-action marker
• chọn = verb
• áo màu xanh = object
• cho bữa tiệc tối nay = purpose phrase

So the structure is very natural and standard Vietnamese.