Nhịn ăn tối nay; xét nghiệm vào ngày mai.

Nhịn ăn is a set phrase meaning to fast or to refrain from eating. It is stronger and more specific than just không ăn (not eat).

In medical contexts, nhịn ăn is very common for instructions before surgery, bloodwork, scans, and so on.

• nhịn ăn = fast / avoid food
• không ăn = not eat
• bỏ bữa = skip a meal

So in this sentence, nhịn ăn sounds like a medical instruction, not just a casual choice.

Usually, it means fast tonight, not just skip dinner.

That is because nhịn ăn is understood as refraining from eating for a period of time. In a hospital or clinic context, it normally means do not eat tonight before tomorrow’s test.

If someone specifically wanted to say skip dinner tonight, they would often say something clearer, such as:

• không ăn tối nay
• bỏ bữa tối nay

So the medical context strongly pushes the meaning toward fasting, not merely not having dinner.

Vietnamese often leaves out the subject when it is obvious from context.

In an instruction like this, the understood subject is basically you. So:

• Nhịn ăn tối nay
  = You should fast tonight
• Xét nghiệm vào ngày mai
  = You will have testing tomorrow / The test is tomorrow

This kind of omission is very common in notices, medical instructions, recipes, and everyday speech.

Yes. It is a very short, note-like instruction.

Vietnamese often uses bare verbs for concise commands or instructions, especially in formal or practical settings.

Compare:

• Nhịn ăn tối nay.
  concise instruction
• Hãy nhịn ăn tối nay.
  a bit more explicit, like please fast tonight
• Phải nhịn ăn tối nay.
  stronger obligation, like must fast tonight
• Đừng ăn tối nay.
  literally don’t eat tonight, but less idiomatic than nhịn ăn in a medical context

So the original sounds natural as a brief instruction.

It can function as either, depending on context.

• As a verb, xét nghiệm means to test, to run tests, or to examine medically
• As a noun, it can mean a test or medical testing

In this sentence, it is a little elliptical, so English may express it in different ways:

• testing tomorrow
• the test is tomorrow
• you will be tested tomorrow
• you will have a test tomorrow

Vietnamese often allows this kind of short, context-dependent phrasing.

Yes, it could just say ngày mai.

The word vào is a preposition often used before time expressions, roughly like on or in, but in many cases it is optional.

So these are all possible:

• xét nghiệm vào ngày mai
• xét nghiệm ngày mai
• ngày mai xét nghiệm

In everyday speech, ngày mai or even just mai is often enough.
Vào ngày mai sounds a bit more formal or written.

Because Vietnamese time expressions are fairly flexible.

Here, the structure is:

• nhịn ăn
  • tối nay
• xét nghiệm
  • vào ngày mai

This is completely natural. But you could also move the time to the front:

• Tối nay nhịn ăn; ngày mai xét nghiệm.

Both are understandable. Putting the time after the verb can feel like instruction first, timing second, while putting it first can feel like time frame first.

It separates two closely related instructions:

• Nhịn ăn tối nay
• xét nghiệm vào ngày mai

It works a lot like a semicolon in English medical or administrative writing: short, connected statements with no need for a full explanatory sentence.

Other punctuation would also be possible depending on style:

• Nhịn ăn tối nay. Xét nghiệm vào ngày mai.
• Nhịn ăn tối nay, xét nghiệm vào ngày mai.
• written on separate lines in a notice

The semicolon gives it a neat, formal, instruction-sheet feel.

Vietnamese often does not mark tense when the time is already clear from context.

Here, ngày mai already tells you the action is in the future. Because of that, sẽ is not necessary.

Compare:

• Xét nghiệm vào ngày mai.
  Testing tomorrow / The test is tomorrow
• Sẽ xét nghiệm vào ngày mai.
  Will be tested tomorrow / Will do testing tomorrow

The version without sẽ is shorter and very natural in instructions.

Yes. The original is very concise, like something written on a hospital note.

A fuller version might be:

• Tối nay bạn cần nhịn ăn; ngày mai bạn sẽ được xét nghiệm.
• Tối nay hãy nhịn ăn. Ngày mai sẽ xét nghiệm.
• Tối nay không được ăn để ngày mai xét nghiệm.

These versions make the relationships clearer, but the original is more compact and efficient.

Yes, especially in a medical or administrative context.

It sounds like a brief written instruction rather than casual conversation. A doctor, nurse, clinic form, or hospital sign could use this kind of wording.

In everyday speech, people might say something slightly fuller, such as:

• Tối nay bạn nhịn ăn nhé, mai xét nghiệm.
• Tối nay đừng ăn gì, ngày mai đi xét nghiệm.

So the original is natural, but it belongs more to instructional/formal note style than to relaxed conversation.