Cô ấy hỏi giáo viên bao giờ phải làm xong bài tập, nhưng rồi lại quên vì đang nghĩ về một vấn đề khác.

Vietnamese pronouns often come from kinship or social terms, not a fixed set like English I / you / he / she.

• cô ấy = that woman / she
• cô can be a pronoun for a woman, often with a nuance of adult female identity
• ấy helps point to a previously known person: that ..., which often corresponds to English she in context

So cô ấy is a very normal way to say she.

Vietnamese usually does not have obligatory tense marking the way English does. Time is often understood from context.

In this sentence:

• hỏi can mean asked
• quên can mean forgot
• đang nghĩ shows an ongoing action: was thinking

Even without a past tense ending, the sequence of events and context make the sentence clearly past in meaning.

If needed, Vietnamese could add time words like hôm qua (yesterday) or aspect markers in some contexts, but they are not required here.

In Vietnamese, hỏi directly takes the person being asked.

• hỏi giáo viên = ask the teacher

English uses ask someone too, but when English says ask the teacher when..., Vietnamese does the same thing directly, without any extra preposition.

So:

• hỏi giáo viên = asked the teacher
• not something like hỏi đến giáo viên

Bao giờ means when.

In this sentence it introduces an indirect question:

• hỏi giáo viên bao giờ phải làm xong bài tập
• literally: asked the teacher when had to finish the homework

More natural English: asked the teacher when the homework had to be finished or when she had to finish the homework.

A useful point:

• Bao giờ can be used in direct questions too: Bao giờ bạn đi? = When are you going?

Here it appears inside a larger sentence, so it is an embedded/indirect question.

Vietnamese often omits subjects when they are understood from context.

Here, the understood subject is the same person as cô ấy. So:

• Cô ấy hỏi giáo viên bao giờ phải làm xong bài tập
• literally: She asked the teacher when [she] had to finish the homework

Vietnamese avoids repeating pronouns when the meaning is already clear.

If you repeated the subject, it could become heavier than necessary:

• Cô ấy hỏi giáo viên bao giờ cô ấy phải làm xong bài tập This is possible, but less natural unless you need emphasis or clarity.

Phải means must / have to.

So:

• phải làm xong bài tập = have to finish the homework

It expresses obligation, not future tense.

Compare:

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

In your sentence, phải tells us she was asking about the required deadline.

Xong adds the idea of completion.

• làm bài tập = do the homework
• làm xong bài tập = finish doing the homework / do the homework completely

This is a very common Vietnamese pattern:

• ăn xong = finish eating
• đọc xong = finish reading
• viết xong = finish writing

So làm xong is not just do; it is specifically do and complete.

Because xong functions like a resultative complement. It comes after the main verb to show the result of the action.

Pattern:

• verb + xong

Examples:

• làm xong = finish doing
• học xong = finish studying
• rửa xong = finish washing

This is a very common Vietnamese structure, and English speakers often need to get used to the idea that the completion word comes after the verb.

These words work together to show a turn in events.

• nhưng = but
• rồi = then / afterward
• lại = often again, but here it adds a sense like ended up, after all, or contrary to expectation

So:

• nhưng rồi lại quên = but then she forgot / but then ended up forgetting

The word lại here does not necessarily mean she forgot repeatedly. It often marks an unexpected reversal or contrast with what came before.

No. Lại has several common uses.

1. Again

   • Anh ấy lại đến. = He came again.

2. A contrastive or unexpected turn

   • Tưởng nhớ, ai ngờ lại quên.
   • One would think she'd remember, but she actually forgot.

In your sentence, it is the second use. It gives the feeling of but then she went and forgot.

Vietnamese often leaves out an object if it is obvious from context.

Here, what she forgot is understood: the answer to the question, or the matter she had just asked about.

So quên alone can mean:

• forgot
• forgot about it
• forgot the answer

You do not need to state the object if the listener can infer it.

Vì means because.

• vì đang nghĩ về một vấn đề khác
• because [she] was thinking about another issue

It introduces the reason for forgetting.

Common pattern:

• ..., vì + clause
• ..., because + clause

Đang marks an action as ongoing or in progress.

• nghĩ = think
• đang nghĩ = be thinking / be in the middle of thinking

In this context, it corresponds well to English was thinking, because it explains what was going on at that moment.

So:

• vì đang nghĩ về một vấn đề khác = because she was thinking about another issue

No. Vietnamese can often rely on context, but đang is commonly used when you want to make the progressive meaning clear.

Compare:

• Cô ấy nghĩ về chuyện đó. = She thinks / thought about that.
• Cô ấy đang nghĩ về chuyện đó. = She is / was thinking about that.

In your sentence, đang is helpful because it explains the background action that caused the forgetting.

Because nghĩ về means think about.

• nghĩ alone = think
• nghĩ về + noun/topic = think about + noun/topic

Examples:

• nghĩ về gia đình = think about family
• nghĩ về công việc = think about work

So:

• đang nghĩ về một vấn đề khác = was thinking about another issue

It is:

• một = one / a
• vấn đề = issue, problem, matter
• khác = other, different, another

So:

• một vấn đề khác = another issue / a different problem

Notice that khác comes after the noun:

• vấn đề khác = another/different issue

This is normal in Vietnamese. Modifiers like khác often follow the noun.

Vietnamese noun phrases are structured differently from English.

English:

• another problem
• different issue

Vietnamese:

• vấn đề khác
• literally: issue different/other

This post-noun placement is very common:

• người khác = other person / someone else
• việc khác = another thing
• ngày khác = another day

Yes. Vấn đề is flexible.

Depending on context, it can mean:

• problem
• issue
• matter
• sometimes question/topic

In your sentence, another issue or something else fits naturally, because the focus is that her mind was on a different matter.

It has two main parts joined by nhưng (but):

1. Cô ấy hỏi giáo viên bao giờ phải làm xong bài tập

   • She asked the teacher when she had to finish the homework

2. nhưng rồi lại quên vì đang nghĩ về một vấn đề khác

   • but then forgot because she was thinking about something else

So the overall pattern is:

• [main clause 1], nhưng [main clause 2] vì [reason]

This is a very common Vietnamese sentence structure.

Yes, but it is usually not necessary.

For example:

• Cô ấy hỏi giáo viên bao giờ cô ấy phải làm xong bài tập, nhưng rồi cô ấy lại quên vì cô ấy đang nghĩ về một vấn đề khác.

This is grammatical, but it sounds repetitive. Vietnamese normally drops repeated subjects when they are obvious.

The original sentence is more natural because it is smoother and less repetitive.

It is an indirect question embedded inside the larger sentence.

Direct question:

• Bao giờ phải làm xong bài tập?
• When does it have to be finished?

Embedded after hỏi:

• Cô ấy hỏi giáo viên bao giờ phải làm xong bài tập.
• She asked the teacher when it had to be finished.

Vietnamese does not need a special word order change here the way English sometimes does.

Yes. Vietnamese does not always mark definiteness the way English does.

• giáo viên can mean a teacher, the teacher, or simply teacher/teachers depending on context.

In this sentence, English naturally uses the teacher because a specific teacher is understood.

This is common in Vietnamese: context does a lot of work that English articles (a, an, the) do explicitly.

Classifiers are common in Vietnamese, but they are not used before every noun in every situation.

Here:

• giáo viên appears without a classifier because it is just the object of hỏi
• một vấn đề khác also works naturally without a separate classifier, since một directly modifies vấn đề

Some nouns commonly appear with classifiers in certain constructions, but not every noun phrase requires one. Learners often overexpect classifiers because Vietnamese textbooks emphasize them early on.

It is natural and grammatically solid. It uses several very common Vietnamese patterns:

• omitted repeated subject
• hỏi + person + indirect question
• phải + verb
• verb + xong
• nhưng rồi lại...
• vì đang...

So it is a good sentence for learning real Vietnamese structure.