chinguga naeil susureul bada.

친구가 marks friend as the subject of the sentence.

In Korean, 가/이 is the subject marker, while 는/은 is the topic marker. In a simple sentence like this, 가 often sounds natural when you are just stating a fact:

• 친구가 내일 수술을 받아.
  My friend is having surgery tomorrow.

If you used 친구는, it would feel more like you are setting up my friend as the topic, often with some contrast or added context:

• 친구는 내일 수술을 받아.
  As for my friend, he/she is having surgery tomorrow.

So 가 here is the more neutral subject-focused choice.

Yes, it often can.

Korean very often leaves out possessives like my, your, and his/her when the meaning is obvious from context. So 친구가 can mean:

• a friend
• my friend
• sometimes just the friend we’re talking about

In real conversation, Korean speakers do not always say 내 친구 unless they want to be extra clear or natural in a specific context.

So:

• 내 친구가 내일 수술을 받아. = My friend is having surgery tomorrow.
• 친구가 내일 수술을 받아. = also very commonly understood as My friend is having surgery tomorrow, depending on context.

Literally, 받다 means to receive.

So 수술을 받아 literally means to receive surgery. That is the natural Korean way to say that someone is undergoing or having surgery as a patient.

This is different from English, where we usually say:

• have surgery
• undergo surgery

In Korean:

• 의사가 수술을 해요. = The doctor performs surgery.
• 환자가 수술을 받아요. = The patient receives/undergoes surgery.

So in your sentence, the friend is the patient, which is why 받다 is used.

Because 수술 is the object of 받다.

Since 받다 means to receive, the thing being received is marked as the object:

• 수술을 받다 = to receive surgery
• 치료를 받다 = to receive treatment
• 도움을 받다 = to receive help

So even though English does not say receive a surgery very often, Korean grammar treats 수술 as the thing being received, so it takes 을/를.

Grammatically, 받아 is in the plain present-style form, but in Korean the present form is often used for a scheduled or planned future event when there is a time word like 내일.

So:

• 친구가 내일 수술을 받아.

is understood as:

• My friend is having surgery tomorrow
• My friend will undergo surgery tomorrow

Because 내일 already gives the time, Korean does not need a separate future ending here.

This is very common in Korean.

받아 is in the casual, non-polite speech style.

You would use it with:

• close friends
• younger people
• people you speak casually with

More polite versions would be:

• 친구가 내일 수술을 받아요.
  polite casual
• 친구가 내일 수술을 받습니다.
  formal/polite

So the sentence as written sounds like something you would say in casual conversation.

Yes. Korean word order is flexible as long as the particles make the roles clear.

These are all possible:

• 친구가 내일 수술을 받아.
• 내일 친구가 수술을 받아.
• 친구가 수술을 내일 받아.
  possible, though less neutral

The most natural order is usually to put the time expression earlier in the sentence, but Korean allows movement for emphasis or flow.

So 친구가 내일 수술을 받아 is perfectly natural, and 내일 친구가 수술을 받아 is natural too.

Usually, not if you mean the friend is the patient.

수술하다 means to perform surgery or sometimes more generally to do surgery. That normally fits the doctor, not the patient.

Compare:

• 의사가 내일 수술해.
  The doctor performs surgery tomorrow.

• 친구가 내일 수술을 받아.
  My friend undergoes surgery tomorrow.

So if your friend is the one being operated on, 수술을 받다 is the correct expression.

Because Korean often does not need one.

If the sentence already has a time word like 내일 (tomorrow), the future meaning is usually clear from context. Korean commonly uses the present form for future events that are planned or expected.

So instead of saying something equivalent to will receive surgery, Korean simply says:

• 내일 수술을 받아

The meaning is still future because 내일 makes it clear.

A very natural polite version is:

• 친구가 내일 수술을 받아요.

If you want to be extra clear that you mean my friend, you can say:

• 제 친구가 내일 수술을 받아요.
  polite, humble my friend
• 내 친구가 내일 수술을 받아요.
  casual my friend

And in a more formal situation:

• 친구가 내일 수술을 받습니다.

So the original sentence is casual, and 받아요 is the most straightforward polite upgrade.