tosyokan ni iku tabi ni, onazi seki ga aite iru ka douka kakuninsimasu.

たびに means every time or whenever.

So:

• 図書館に行くたびに = every time I go to the library

A useful pattern is:

• dictionary form verb + たびに
• noun + の + たびに

Examples:

• 会うたびに = every time I meet someone
• 旅行のたびに = every time I travel

It expresses something that happens repeatedly whenever the first action/event happens.

Before たびに, verbs normally use the dictionary form.

So:

• 行くたびに = every time (someone) goes

This is just the standard grammar pattern. It does not mean the sentence is casual overall. The sentence still ends politely with 確認します.

You can think of it like this:

• subordinate clause: 図書館に行くたびに
• main clause: 確認します

Japanese often mixes a plain-form clause with a polite ending in the same sentence.

The に after 図書館 marks the destination of 行く.

• 図書館に行く = go to the library

This is the normal particle used with 行く, 来る, and 帰る when showing where someone goes.

You could also sometimes see へ instead:

• 図書館へ行く

But に is more common in everyday speech, and it feels a bit more direct/specific as a destination marker.

That に is part of the grammar expression たびに. It is not the same に as the destination marker in 図書館に.

So the sentence contains two different に particles with different jobs:

• 図書館に = to the library
• たびに = every time / whenever

This is very common in Japanese: the same particle can appear more than once in a sentence for different grammatical reasons.

Here, 空いている means is empty, is available, or is open/unoccupied.

In this sentence:

• 同じ席が空いている = the same seat is available / unoccupied

This comes from the verb 空く（あく）, an intransitive verb meaning things like:

• to become empty
• to open up
• to be available

The ～ている form here often describes a state, not an action in progress.

So 空いている does not mean the seat is in the middle of opening. It means the seat is currently in the state of being open/empty.

Compare:

• ドアが開いている = the door is open
• 席が空いている = the seat is empty/available

が marks the thing being checked for a condition.

The condition here is:

• 同じ席が空いている = whether the same seat is available

So 同じ席 is the subject of 空いている.

Why not を?

• を usually marks a direct object of an action verb.
• But 空いている is describing a state, and 席 is the thing that is in that state.

Why not は?

• は would add a contrastive or topic nuance.
• が is more neutral and natural here when stating the condition being checked.

かどうか means whether or not.

So:

• 同じ席が空いているかどうか確認します
• = I check whether the same seat is available

It turns the whole clause before it into something like an embedded yes/no question.

Structure:

• [clause] + かどうか + 確認する
• = check whether [clause]

Other examples:

• 行けるかどうか分かりません。
  = I don’t know whether I can go.

• 本当かどうか調べます。
  = I’ll investigate whether it’s true.

Yes, you often can.

• 同じ席が空いているか確認します
• 同じ席が空いているかどうか確認します

Both are natural.

The difference is:

• か alone = whether
• かどうか = whether or not

In many sentences, the meaning is almost the same. かどうか can sound a bit more explicit or complete, but neither is strange here.

確認します means confirm, check, or make sure. It sounds more deliberate than just 見る.

Compare:

• 見ます = look
• 確認します = check/confirm

In this sentence, the speaker is not just casually looking at the seat. They are actively checking whether it is available. So 確認します fits better.

You could also hear:

• チェックします = check
  This is common too, but more casual and more like a loanword expression.

確認します sounds natural, standard, and slightly more formal.

同じ席 means the same seat.

It implies that the speaker has a particular seat in mind—probably a favorite seat or the seat they usually use.

So the sentence suggests something like:

• Every time I go to the library, I check whether that same seat is free.

It does not mean just any identical kind of seat. It usually means one specific seat that the speaker wants.

A helpful breakdown is:

• 図書館に行くたびに、
  every time I go to the library,

• 同じ席が空いているかどうか
  whether the same seat is available

• 確認します。
  I check.

So the overall structure is:

• [time/condition clause], [embedded clause] + 確認します

Japanese often puts the context first and the main action at the end.

A very literal order would be:

• Every time I go to the library, whether the same seat is available, I confirm.

That sounds unnatural in English, but it shows how Japanese builds the sentence.