sueobi sijakdoegi jeone seonsaengnimkke insahaeyo.

-기 전에 means before doing or before something happens.

It is made from:

• the verb stem
• -기, which turns the verb into something noun-like
• 전에, meaning before

So:

• 시작되다 = to begin
• 시작되기 전에 = before it begins

In this sentence, 수업이 시작되기 전에 means before class begins.

Because 수업 is the subject of 시작되다.

The clause 수업이 시작되기 전에 literally means before the class begins.
Here, the thing that begins is 수업, so it gets the subject marker 이/가.

Compare:

• 수업이 시작되다 = class begins
• 수업을 시작하다 = to start class

So 이 is used because the sentence is talking about the class beginning, not someone actively starting it.

This is a very common learner question.

• 시작되다 means to begin / to be started
• 시작하다 means to start

In this sentence, the focus is on the class beginning, so 수업이 시작되기 전에 is natural.

If you say 수업을 시작하기 전에, that means before starting class, which focuses more on a person doing the starting.

So the difference is:

• 수업이 시작되기 전에 = before class begins
• 수업을 시작하기 전에 = before starting class

Both can make sense, but they are slightly different in focus.

께 is an honorific particle meaning to.

It is the polite/respectful version of 에게. Since 선생님 is someone you should show respect to, 께 is a very natural choice.

So:

• 선생님께 인사해요 = greet the teacher

Compare:

• 친구에게 인사해요 = greet a friend
• 선생님께 인사해요 = greet the teacher

Using 께 helps match the respectful tone of talking about a teacher.

Yes, grammatically you can, but 선생님께 sounds more respectful and more natural in this context.

• 선생님께 = respectful, preferred for teachers
• 선생님에게 = neutral
• 선생님한테 = more casual

Because a teacher is someone you usually speak about respectfully, 께 is the best fit here.

Actually, both are possible.

• 인사해요
• 인사를 해요

They both mean greet / say hello.

What is happening is:

• 인사 is a noun meaning greeting
• 하다 means to do
• 인사하다 becomes a single verb: to greet

So Korean often allows both:

• the combined verb form: 인사하다
• the noun + 하다 form: 인사를 하다

In everyday speech, 인사해요 is very common and natural.

Because Korean normally puts:

1. background information first
2. the main verb at the end

So the sentence is organized like this:

• 수업이 시작되기 전에 = before class begins
• 선생님께 = to the teacher
• 인사해요 = greet

This is very typical Korean word order. English often says:

• We greet the teacher before class begins

Korean often prefers:

• Before class begins, to the teacher, greet

It can feel backward at first, but it is a normal Korean pattern.

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

So 인사해요 could mean:

• I greet the teacher
• we greet the teacher
• you should greet the teacher

The exact subject depends on the situation.

In a textbook sentence like this, it often means something like:

• We greet the teacher before class begins or
• You greet the teacher before class begins

Korean does not need to state the subject every time if it is understood.

해요 is the polite present-tense form of 하다.

So:

• 인사하다 = to greet
• 인사해요 = greet / do greeting, in a polite style

This is the standard -아요/어요 polite style, which is very common in conversation and textbooks.

You might also see:

• 인사합니다 = more formal
• 인사해 = casual

So 인사해요 is polite but not stiff.

Yes. A very common shorter version is:

• 수업 시작 전에

This also means before class starts.

Compare:

• 수업이 시작되기 전에 = a full verb clause, slightly more explicit
• 수업 시작 전에 = shorter and very natural

Both are good. The version in your sentence is just a bit fuller grammatically.

It can mean more than just a simple hello.

인사하다 can refer to:

• saying hello
• greeting someone politely
• bowing
• giving a formal greeting

So in a classroom context, 선생님께 인사해요 could include both speech and action, such as:

• saying 안녕하세요
• bowing to the teacher

That is why 인사하다 is often best understood as to greet, not just to say hello.