zyugyoutyuu, nemukute akubi wo site simaimasita.

授業中 means during class or in the middle of class.

Here, 中（ちゅう） means during / in the middle of when attached to a noun.

• 授業 = class, lesson
• 授業中 = during class

This pattern is very common:

• 会議中 = during a meeting
• 仕事中 = while working / during work
• 食事中 = during a meal

So 授業中、 sets the scene: while class was going on.

The comma helps separate the time/situation phrase from the main statement.

So the sentence is easier to read as:

• 授業中、 = during class,
• 眠くて = I got sleepy / being sleepy,
• あくびをしてしまいました = I ended up yawning

In Japanese, commas are often used more flexibly than in English. This comma is natural because 授業中 is an introductory phrase.

眠い is an i-adjective meaning sleepy.

To connect it to what happened next, Japanese changes 眠い to its て-form:

• 眠い → 眠くて

This works like being sleepy, ... or because I was sleepy, ...

So:

• 眠くて あくびをしてしまいました = I got sleepy and ended up yawning = Because I was sleepy, I ended up yawning

You cannot say 眠いで here. For i-adjectives, the connector is 〜くて.

Compare:

• 高い → 高くて
• 暑い → 暑くて
• 眠い → 眠くて

It can feel like either, depending on context.

The て-form often connects ideas in a broad way, and in this sentence 眠くて can naturally suggest:

• I was sleepy, and...
• Because I was sleepy,...

Both are reasonable.

So the sentence does not force a sharp distinction. It simply links being sleepy with yawning. In natural English, because I was sleepy is often the most natural interpretation.

In Japanese, あくび is a noun, meaning a yawn.

To say to yawn, Japanese commonly uses:

• あくびをする

This is a very common pattern:

• 勉強をする = to study
• 運動をする = to exercise
• 電話をする = to make a phone call
• あくびをする = to yawn

So を marks あくび as the thing being done with する.

Yes. In casual or natural speech, あくびした is very common and sounds perfectly normal.

Both are grammatical:

• あくびをした
• あくびした

The version with を can sound a little more explicit or slightly more careful, while dropping を is very common in everyday Japanese with する-verbs.

In this sentence, あくびをしてしまいました is a polite, clear form.

〜てしまいました is the polite past form of 〜てしまう.

This grammar often adds one of these feelings:

1. completion — doing something completely
2. regret / unintended action / “oops” feeling

In this sentence, the second feeling is the important one.

• あくびをしてしまいました = I ended up yawning = I yawned without meaning to / unfortunately yawned

Because yawning in class is a little embarrassing or not ideal, しまいました gives that sense of I did it, and I wish I hadn’t or it happened unintentionally.

These are very different:

• していました = was doing
• してしまいました = ended up doing / did unfortunately / did completely

So:

• あくびをしていました = I was yawning or I was in the middle of yawning
• あくびをしてしまいました = I ended up yawning

The sentence is not describing an ongoing action. It describes a completed action with a slight sense of regret or unintendedness.

This is an important nuance difference.

• あくびをしました = I yawned
• あくびをしてしまいました = I ended up yawning / I accidentally yawned / I yawned and it was unfortunate

The second version adds emotion or attitude. It suggests something like:

• it wasn’t ideal
• it happened unintentionally
• the speaker feels awkward, embarrassed, or regretful about it

Since yawning in class can seem rude or embarrassing, 〜てしまいました fits very naturally.

The final verb is してしまいました, which is:

• する → してしまう → してしまいました

This makes the sentence polite and past.

Japanese often marks tense and politeness mainly at the end of the sentence. Earlier parts like 授業中 and 眠くて do not need separate past marking here.

So even though 眠くて itself does not look past, the whole sentence is understood as past because the final verb is past.

The subject is understood from context, and it is most naturally I.

Japanese often omits subjects when they are obvious. So instead of saying:

• 私は授業中、眠くてあくびをしてしまいました。

Japanese often just says:

• 授業中、眠くてあくびをしてしまいました。

This is very natural. If the speaker is talking about their own experience, I is already clear.

To some extent, yes. Japanese word order is more flexible than English, especially for phrases like time expressions.

For example, these are all understandable:

• 授業中、眠くてあくびをしてしまいました。
• 眠くて、授業中にあくびをしてしまいました。

But the original sentence sounds very natural because it first gives the setting:

• during class

and then explains what happened.

So the original order is a smooth, natural way to tell the story.

Yes, 授業中に is also possible.

• 授業中、眠くてあくびをしてしまいました。
• 授業中に、眠くてあくびをしてしまいました。

Both are natural.

The version without に is very common when 授業中 is used as a set time-context phrase, almost like during class as an adverbial expression.

Using に can make the time relationship a little more explicit, but it is not necessary here.

It does both at the same time.

Japanese often uses the て-form in a way that leaves the relationship slightly broad. Here 眠くて tells us:

• the speaker was sleepy
• that sleepiness is connected to the yawning
• it likely explains why the yawning happened

So it describes the condition and also functions like the reason. Japanese often allows that kind of natural overlap.