kare ha henzi wo matte iru aida, irairasite heya wo urourosite ita.

Both are possible, but they mean slightly different things.

• 待つあいだ = during the time of waiting in a more general sense
• 待っているあいだ = while he is in the state of waiting

Because 待つ is often used as an ongoing action, Japanese commonly uses 〜ている here to show that the person is actively in the middle of waiting.

So 返事を待っているあいだ sounds very natural for while he was waiting for a reply.

あいだ（間） means while, during, or for the period that.

In this sentence:

• 返事を待っているあいだ = while he was waiting for a reply

It marks a stretch of time during which the main action happened.

So the structure is:

• A あいだ、B
• While A, B

Here:

• A = he was waiting for a reply
• B = he was getting irritated and pacing around the room

This is a very common question.

• 〜あいだ focuses on something that continues throughout that period
• 〜あいだに often focuses on something that happens at some point within that period

Compare:

• 日本にいるあいだ、日本語を勉強した。
  = While I was in Japan, I studied Japanese.
  Studying continued during that time.

• 日本にいるあいだに、富士山に登った。
  = While I was in Japan, I climbed Mt. Fuji.
  This happened once during that period.

In your sentence, the irritation and pacing are ongoing during the waiting, so あいだ fits well.

This is about tense and aspect.

• 待っている is inside the time expression 〜あいだ, and Japanese often keeps this part in a kind of non-past form even when the whole sentence is talking about the past.
• うろうろしていた is the main predicate of the sentence, so its tense shows the overall past time.

So the sentence is describing a past situation:

• While he was waiting for a reply, he was pacing around the room in irritation.

You can think of it like this:

• subordinate clause: 返事を待っているあいだ
• main clause: いらいらして部屋をうろうろしていた

The main clause is what anchors the sentence in the past.

いらいらする means to become irritated, to feel frustrated, to feel impatient, or sometimes to feel edgy/restless.

In this sentence, it probably means something like:

• feeling impatient
• feeling irritated
• being restless with frustration

So いらいらして connects that feeling to the next action:

• being irritated/restless, he paced around the room

It is not just simple anger. It often suggests nervous frustration or impatience.

The て-form is connecting ideas.

Here it links:

• いらいらして
• 部屋をうろうろしていた

So the meaning is something like:

• He was irritated and was pacing around the room
• Feeling irritated, he paced around the room

The て-form can connect actions, states, reasons, or accompanying circumstances. Here it gives the emotional state that goes together with the pacing.

Yes, いらいらしながら would also be natural.

Compare:

• いらいらして部屋をうろうろしていた
• いらいらしながら部屋をうろうろしていた

The difference is small:

• 〜て is a general connector
• 〜ながら emphasizes that the emotional state and action happen simultaneously

So:

• いらいらして = being irritated, ...
• いらいらしながら = while feeling irritated, ...

Both work, but the original sentence is very natural.

うろうろする means to wander around, to pace about, or to move around restlessly without settling down.

It is often used when someone:

• does not stay in one place
• moves back and forth
• seems uncertain, nervous, or aimless

In this sentence, it gives the image of someone pacing around the room because he is impatient and uneasy.

It is also a mimetic/onomatopoeic-style expression, which is very common in Japanese.

This is another very common learner question.

With verbs of movement, を can mark the space that someone moves through.

So:

• 部屋をうろうろしていた = he was wandering/pacing around the room

Here, を shows the area along which the movement takes place.

Compare:

• 道を歩く = walk along the road
• 公園を走る = run through the park
• 部屋をうろうろする = pace around the room

If you used 部屋で, it would focus more on the room as the location where the action happens, but 部屋をうろうろする is the standard way to express moving around within that space.

Yes, 彼 could often be omitted in Japanese if the subject is already clear from context.

Japanese frequently leaves out subjects when they are understood. So a sentence like this could naturally appear without 彼は.

However, 彼は may be included to:

• make the subject explicit
• contrast him with someone else
• help the sentence stand alone clearly

So both are possible, but including 彼は makes it clearer, especially in an isolated example sentence.

返事 means reply, response, or answer, depending on context.

In this sentence, reply or response is probably the best fit because it suggests he is waiting for someone to get back to him.

Examples:

• メールの返事 = a reply to an email
• 返事を待つ = wait for a reply

So 返事を待っている is a very common expression meaning to be waiting for a reply/response.

Because 待つ is a transitive verb here, and the thing being waited for is marked with を.

So:

• 返事を待つ = to wait for a reply
• バスを待つ = to wait for the bus
• 友達を待つ = to wait for a friend

This is different from English, where we usually say wait for something. In Japanese, the thing waited for often takes を directly.

The whole sentence is in the past because the main predicate is:

• うろうろしていた

This is past progressive/past continuous in feeling:

• was pacing around
• had been pacing around

So the full sentence describes an ongoing scene in the past:

• while he was waiting for a reply, he was restlessly pacing around the room

Japanese often uses this kind of structure to paint a scene rather than just report a single completed action.

It usually suggests repeated or continuous restless movement.

So it is not just:

• he moved once around the room

It is more like:

• he kept pacing
• he wandered back and forth
• he was restlessly moving around

Because うろうろする already suggests aimless or repeated movement, and していた adds the sense of an ongoing past action.

So the image is quite vivid: he could not sit still.

The comma helps readability, but it is not always required.

Here it separates the time clause from the main clause:

• 返事を待っているあいだ、
• いらいらして部屋をうろうろしていた。

It works like a pause in English after While he was waiting for a reply, ...

Japanese punctuation is somewhat flexible, but this comma is very natural and makes the sentence easier to read.

It describes one connected scene.

The parts work together like this:

• 返事を待っているあいだ = the time frame
• いらいらして = the emotional state
• 部屋をうろうろしていた = the visible action

So rather than sounding like disconnected events, it paints a single moment:

• during the waiting, he was irritated
• because of that emotional state, he was pacing around the room

This kind of layered description is very common in Japanese narrative style.