Buổi tối rất lạnh, nên tôi phải đóng cửa sổ.

Vietnamese usually does not use a verb like to be before adjectives.

So:

• Buổi tối rất lạnh = The evening/night is very cold
• literally: evening very cold

This is normal in Vietnamese. Adjectives can act like the main predicate by themselves.

Compare:

• Trời lạnh. = The weather is cold.
• Cô ấy đẹp. = She is beautiful.

You do not say là here. Là is mainly used to link two nouns, not before adjectives.

Buổi tối means in the evening / at night / the evening time.

• buổi = a period/session/time of day
• tối = evening, night

So buổi tối sounds a little more complete and time-expression-like.

In many cases, tối alone can also be used:

• Tối rất lạnh.
• Buổi tối rất lạnh.

Both can work, but buổi tối feels a bit fuller and slightly more formal or explicit.

In Vietnamese, degree words like rất (very) come before the adjective.

So:

• rất lạnh = very cold
• rất đẹp = very beautiful
• rất khó = very difficult

This is the normal word order:

• adverb of degree + adjective

Nên here means so, therefore, or that’s why.

It connects the two ideas:

• Buổi tối rất lạnh = The evening is very cold
• nên tôi phải đóng cửa sổ = so I have to close the window

So the pattern is:

• cause + nên + result

Example:

• Trời mưa, nên tôi ở nhà.
  It’s raining, so I stay home.

In everyday Vietnamese, people sometimes omit nên if the relationship is already clear, but using it is very natural.

The comma separates the reason from the result.

• Buổi tối rất lạnh, nên tôi phải đóng cửa sổ.

This is similar to English:

• It’s very cold at night, so I have to close the window.

In casual writing, punctuation may vary, but the comma here is standard and helpful.

Phải means must, have to, or need to, depending on context.

In this sentence:

• tôi phải đóng cửa sổ = I have to close the window

It expresses necessity. Because it is cold, closing the window is necessary.

A few nuances:

• phải can sound like must / have to
• in many everyday situations, English have to is the most natural translation

Examples:

• Tôi phải đi làm. = I have to go to work.
• Bạn phải học. = You must study / You have to study.

Yes.

• đóng = close / shut
• cửa sổ = window

So:

• đóng cửa sổ = close the window

This is very natural Vietnamese.

Related words:

• mở cửa sổ = open the window
• đóng cửa = close the door
• mở cửa = open the door

Because Vietnamese usually uses the order:

• verb + object

So:

• đóng cửa sổ = close window
• mở cửa = open door
• ăn cơm = eat rice/meal
• đọc sách = read books

This is the normal Vietnamese sentence pattern.

Tôi means I and is a neutral, common first-person pronoun.

So:

• tôi phải đóng cửa sổ = I have to close the window

Yes, another pronoun could be used depending on who is speaking and the social situation:

• mình
• tao
• em
• anh
• chị
• con

Vietnamese pronouns depend a lot on age, relationship, and politeness. Tôi is a safe, standard choice.

Vietnamese does not mark tense the same way English does.

This sentence has no explicit tense marker, so the time is understood from context.

• Buổi tối rất lạnh, nên tôi phải đóng cửa sổ.

Depending on context, it could mean:

• It is very cold in the evening, so I have to close the window.
• It was very cold in the evening, so I had to close the window.
• It will be very cold in the evening, so I will have to close the window.

If needed, Vietnamese can add time words:

• Hôm qua buổi tối rất lạnh... = Yesterday evening was very cold...
• Tối nay rất lạnh... = Tonight is very cold...
• Ngày mai buổi tối sẽ rất lạnh... = Tomorrow evening will be very cold...

Yes, depending on context.

Buổi tối covers the evening/night period rather broadly. English may translate it as:

• in the evening
• at night
• in the nighttime

The exact translation depends on what sounds most natural in English and on the situation being described.

By itself, cửa sổ is usually understood as window, but Vietnamese nouns often do not change form for singular vs. plural.

So context decides:

• đóng cửa sổ = close the window
• in another context, it could mean close the windows

If you want to make plural clearer, you can add a plural marker or number, for example:

• các cửa sổ = the windows
• hai cửa sổ = two windows

In this sentence, the window is the most natural interpretation.

Yes. That would also be natural.

• rất lạnh = very cold
• lạnh quá = so cold / too cold / very cold!

The difference is tone:

• rất lạnh sounds more neutral and descriptive
• lạnh quá sounds more expressive or emotional

So:

• Buổi tối rất lạnh... = calm statement
• Buổi tối lạnh quá... = stronger feeling about the cold

Both are correct, but they do not feel exactly the same.

Yes, but the meaning changes slightly.

• Buổi tối rất lạnh = The evening/night is very cold or It’s very cold in the evening
• Trời rất lạnh = The weather is very cold or It’s very cold

So:

• buổi tối focuses on time
• trời focuses on weather/atmosphere

If you want both ideas, you could say:

• Buổi tối trời rất lạnh, nên tôi phải đóng cửa sổ.
  = In the evening, it’s very cold, so I have to close the window.

That version is also very natural.