Tôi đang kiểm tra lại bài tập thì giáo viên gọi và hỏi vì sao tôi chưa vào lớp học.

đang marks an action as in progress at that moment.

So:

• Tôi kiểm tra lại bài tập = I check / recheck the homework
• Tôi đang kiểm tra lại bài tập = I am checking / rechecking the homework

It often corresponds to English be + -ing, but not always one-to-one. Vietnamese does not have tense in the same way English does, so đang mainly shows ongoing aspect.

In this sentence, lại means again or back over.

• kiểm tra = check
• kiểm tra lại = check again / review / double-check

Very often, verb + lại suggests doing something one more time, often to make sure it is correct.

Examples:

• đọc lại = read again
• xem lại = review, look over again
• nghe lại = listen again

So kiểm tra lại bài tập suggests the speaker was reviewing the homework, not looking at it for the first time.

It can mean any of those depending on context.

Most basically:

• bài = a piece of work / lesson item
• tập = practice

Together, bài tập usually means:

• exercise
• assignment
• homework

In a school context like this one, translating it as homework or assignment is very natural.

Here, thì helps connect two events and gives a sense like:

• when
• just then
• at that moment
• while ... , ...

In this sentence:

• Tôi đang kiểm tra lại bài tập thì giáo viên gọi...

means something like:

• I was reviewing my homework when the teacher called...

This thì is not the same as the very common topic-marker thì in patterns like còn tôi thì.... Here it links events in time.

Yes, very common.

The pattern đang ... thì ... often means:

• was doing X when Y happened
• in the middle of doing X, then Y happened

Examples:

• Tôi đang ăn thì bạn gọi.
  = I was eating when you called.
• Cô ấy đang ngủ thì trời mưa.
  = She was sleeping when it started raining.

So in your sentence, đang sets up an ongoing action, and thì introduces the interrupting event.

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

So giáo viên can mean:

• a teacher
• the teacher
• teacher

The context tells you which one is natural. In this sentence, English usually uses the teacher because it sounds like a specific teacher relevant to the class.

Yes, and that is exactly why both verbs appear.

• gọi = call
• hỏi = ask

So:

• giáo viên gọi và hỏi... = the teacher called and asked...

This is natural Vietnamese. The first verb tells you the teacher contacted the speaker, and the second tells you what the teacher did during that contact.

A very literal breakdown is:

• The teacher called
• and asked why...

Both mean why.

In most contexts, vì sao and tại sao are interchangeable:

• Vì sao bạn đến muộn?
• Tại sao bạn đến muộn?

Both = Why are you late?

A few notes:

• tại sao is probably a bit more common in everyday speech
• vì sao can sound slightly more formal or literary in some contexts
• In this sentence, either would be natural

So you could also say:

• ... hỏi tại sao tôi chưa vào lớp học.

This is a very important difference.

• không = not
• chưa = not yet

So:

• tôi không vào lớp học = I do not enter / I didn’t enter the classroom
• tôi chưa vào lớp học = I have not entered the classroom yet

In your sentence, the teacher is asking why the speaker still had not gone into the class yet, so chưa is exactly the right choice.

Literally:

• vào = go in / enter
• lớp học = classroom / class

So vào lớp học means go into the classroom or join the class.

Depending on context, it can imply either:

• physically entering the classroom
• entering/joining the lesson

If this were an online-class context, vào lớp học could also mean join the class session.

Yes, but the meaning shifts slightly.

• đến lớp = come to class / arrive at class
• vào lớp học = enter the classroom / go into class

So:

• chưa đến lớp focuses on not having arrived
• chưa vào lớp học focuses on not having entered or joined yet

Your sentence suggests the teacher is asking why the speaker has not gone into the class yet, which may imply they are already nearby, late, or not yet connected/joined.

Because Vietnamese often keeps the subject explicit when it helps clarity.

The clause vì sao tôi chưa vào lớp học means why I hadn’t entered the class yet, and tôi clearly marks who the teacher is asking about.

In conversation, Vietnamese sometimes omits pronouns when they are obvious, but here keeping tôi is the normal and clear choice.

Vietnamese keeps a fairly straightforward order here:

• hỏi = ask
• vì sao = why
• tôi = I
• chưa vào lớp học = had not entered the class yet

So the embedded question is literally:

• why + I + not yet entered class

This is similar to English indirect question word order:

• The teacher asked why I hadn’t entered the class yet.

Notice that English indirect questions do not use normal direct-question inversion either:

• direct: Why haven’t you entered?
• indirect: asked why I hadn’t entered

Vietnamese simply keeps its normal clause order after vì sao.

Yes. Vietnamese often relies on context rather than verb tense endings.

This sentence can be understood as past because:

• it describes a sequence of events
• đang ... thì ... often tells a past narrative
• English naturally translates it with past forms: was checking, called, asked

Vietnamese does have time words and markers like:

• đã = past/completed
• đang = ongoing
• sẽ = future

But they do not work exactly like English tenses. Here, the timeline is clear without đã.

Yes. A few natural variants are:

• Tôi đang kiểm tra lại bài tập thì giáo viên gọi hỏi vì sao tôi chưa vào lớp.
• Trong lúc tôi đang kiểm tra lại bài tập, giáo viên gọi và hỏi vì sao tôi chưa vào lớp.
• Tôi đang xem lại bài tập thì giáo viên gọi và hỏi tại sao tôi chưa vào lớp.

These are all close in meaning, but your original sentence is perfectly natural.