sidong beoteungwa josuseok muneul hwaginhaessseupnida.

과 means and and connects the two nouns:

• 시동 버튼과 조수석 문을
• the start button and the passenger-side door

A useful point: 과 is a slightly more formal/literary version of 와.
In everyday speech, people often use:

• 와 / 과
• 하고
• (이)랑

So this sentence sounds natural and a bit formal, which matches the ending -습니다.

Because the two nouns are being treated as one combined object:

• 시동 버튼과 조수석 문 = the start button and the passenger-side door
• then 을 marks that whole phrase as the object

So:

• 시동 버튼과 조수석 문을 확인했습니다
  means checked the start button and the passenger-side door

Korean often puts the case particle only on the last noun in a coordinated phrase like this.

You could also sometimes see both nouns marked in other contexts, but here marking only the final noun is very normal and natural.

확인했습니다 comes from the verb 확인하다, which means to check, to confirm, or to verify.

Breakdown:

• 확인하다 = to check/confirm
• 확인했- = checked / confirmed (past)
• 확인했습니다 = checked / confirmed (formal polite past)

So the sentence is in the formal polite past tense, which is commonly used in announcements, reports, workplace situations, and other formal contexts.

Because 확인하다 has the sense of checking carefully or verifying something.

Compare:

• 보다 = to see, look at, try
• 확인하다 = to check, confirm, make sure

So if someone is inspecting something, making sure it is okay, or verifying its condition, 확인하다 is a better choice.

In this sentence, the speaker is not just looking at the button and the door—they are checking them.

시동 means starting an engine or ignition/start-up.
버튼 means button.

So 시동 버튼 is literally start button or ignition button.

This is a noun + noun combination, which is very common in Korean. The first noun describes the second:

• 시동 버튼 = the button for starting the engine

Literally:

• 조수석 = passenger seat, usually the front passenger seat
• 문 = door

So 조수석 문 means the passenger-side door, usually understood as the front passenger door in a car context.

Even though 조수석 literally contains seat, Korean often uses seat-position words to refer to that side of the car. So this expression sounds natural.

Because Korean very often combines nouns directly without needing a word like of.

So instead of something like:

• the door of the passenger seat

Korean naturally says:

• 조수석 문

This kind of noun-modifying structure is extremely common:

• 차 문 = car door
• 운전석 창문 = driver-side window
• 시동 버튼 = start button

Using 의 here, as in 조수석의 문, is possible in some contexts, but it would usually sound less natural for this kind of everyday compound noun.

Yes, the subject is omitted.

That is very common in Korean. If the subject is already understood from context, Korean often leaves it out.

So this sentence could mean:

• I checked the start button and the passenger-side door.
• We checked the start button and the passenger-side door.
• He/She checked the start button and the passenger-side door.

The exact subject depends on the situation.

Because Korean is basically an SOV language: Subject-Object-Verb.

So the normal order is:

• subject (often omitted)
• object
• verb

In this sentence:

• 시동 버튼과 조수석 문을 = object
• 확인했습니다 = verb

That is the normal Korean pattern.

A few common versions are:

• 시동 버튼과 조수석 문을 확인했어요.
  polite, less formal

• 시동 버튼과 조수석 문 확인했어요.
  conversational, with the object particle sometimes dropped in speech

• 시동 버튼과 조수석 문 확인했어.
  casual/informal

The original 확인했습니다 is the most formal among these.