megane wo tukue no ue ni okippanasi ni site, dekakete simaimasita.

を marks the direct object of the action.

Here, the verb idea is 置く/置きっぱなしにする, so 眼鏡を means the glasses are the thing being left behind.

A literal breakdown is:

• 眼鏡を = the glasses
• 机の上に = on the desk
• 置きっぱなしにして = leaving them there / leaving them as they were placed
• 出かけてしまいました = went out, unfortunately / ended up going out

So 眼鏡 is what was left behind, which is why it takes を.

Because 置く takes に for the place where something is put.

• 机の上に置く = to put something on top of the desk
• 机の上で置く would sound unnatural here

A useful contrast:

• に marks the destination or resulting location of placement
  
  • 本を机の上に置く
• で marks the place where an action happens
  
  • 机の上で勉強する

Since the glasses end up on the desk, に is the natural choice.

置きっぱなし means left sitting there, left as is after putting it down, or left behind without dealing with it.

It comes from:

• 置く = to put
• っぱなし = left in that state; done and then not corrected or finished properly

So 置きっぱなし has the nuance of:

• putting something somewhere
• and then just leaving it there

It often suggests carelessness, forgetfulness, or neglect.

Examples:

• 電気をつけっぱなしにする = to leave the lights on
• ドアを開けっぱなしにする = to leave the door open
• 荷物を置きっぱなしにする = to leave your stuff lying around

In this sentence, it suggests the speaker left their glasses on the desk and went out without taking them.

Because 置きっぱなしにする is the pattern.

Here, 置きっぱなし behaves like a noun-like state, and にする means to make it that way / leave it in that state.

So:

• 置きっぱなしにする = to leave something sitting there
• 開けっぱなしにする = to leave something open
• つけっぱなしにする = to leave something turned on

In your sentence:

• 置きっぱなしにして = leaving it left there, and...

So the second に is not the same as the に in 机の上に.

• 机の上に = location
• 置きっぱなしに = into that state / in that condition

Because 置いて simply means putting/placing something, while 置きっぱなしにして adds the important nuance that it was left there and not taken care of.

Compare:

• 眼鏡を机の上に置いて、出かけました。
  = I put my glasses on the desk and went out.
  This sounds more neutral.

• 眼鏡を机の上に置きっぱなしにして、出かけました。
  = I left my glasses on the desk and went out.
  This suggests the speaker forgot them or carelessly left them behind.

So 置きっぱなし is used because the speaker wants to express more than just the act of placing the glasses down.

It connects the first action/state to the next part of the sentence.

So:

• 置きっぱなしにして、
• 出かけてしまいました。

means something like:

• having left them on the desk,
• I went out.

The て-form often links actions in sequence. Here it shows that first the glasses were left behind, and then the speaker went out.

It can also imply the circumstances under which the second action happened:

• with my glasses left on the desk, I went out

しまいました adds nuance.

Basic meaning:

• 出かけました = went out
• 出かけてしまいました = went out completely / ended up going out / unfortunately went out

In many contexts, 〜てしまう can suggest:

1. completion
   • the action happened fully
2. regret / mistake / unintended result
   • something happened and the speaker feels it was unfortunate

In this sentence, because the speaker left their glasses behind, 出かけてしまいました strongly suggests regret or oops, I went out without them.

So it is not just I went out. It feels more like:

• I ended up going out
• I went out, unfortunately
• I accidentally went out leaving my glasses behind

Not always.

〜てしまう / 〜てしまいました can mean:

1. completed entirely

   • 宿題をやってしまいました。
     = I finished the homework.

2. unfortunate / unintended

   • 財布をなくしてしまいました。
     = I lost my wallet.

In your sentence, it sounds regretful because of the context: leaving your glasses behind is inconvenient.

So the regret is not from 〜てしまいました alone; it comes from the combination of that form and the situation.

Yes, that is a very natural implication.

The sentence does not literally say I forgot my glasses, but it strongly suggests that the speaker left them on the desk and went out without meaning to.

The clues are:

• 置きっぱなし = left there carelessly / left as is
• 〜てしまいました = unfortunate or regrettable result

Together, they create the feeling:

• I left my glasses on the desk and accidentally went out.

So even without saying 忘れました, the sentence naturally implies forgetfulness or carelessness.

Because Japanese often omits the subject when it is obvious from context.

In this sentence, the natural subject is understood to be the speaker:

• (私は) 眼鏡を机の上に置きっぱなしにして、出かけてしまいました。

Japanese frequently leaves out:

• I
• you
• he/she when the listener can infer them.

Since this sentence sounds like the speaker is talking about their own mistake or regret, I is the most natural interpretation.

Here it means a pair of glasses.

In Japanese, nouns usually do not show singular/plural the way English does. So 眼鏡 can refer to glasses in a general sense, and context tells you how to understand it.

In this sentence, the normal interpretation is:

• my glasses
• a pair of glasses

Not several different pairs.

Yes, 〜っぱなし has a somewhat conversational feel, but it is extremely common and natural.

The sentence mixes:

• a conversational expression: 置きっぱなし
• with polite verb endings: しまいました

That combination is perfectly normal in everyday polite speech.

So this sentence sounds natural and polite overall, but not stiff or formal in a business-report kind of way.

Grammatically, that idea is not completely wrong, but it is too literal for natural English.

The structure 置きっぱなしにする does literally involve にする, which often means to make something into a certain state. But as a set phrase, it is better understood as:

• to leave something lying there
• to leave something behind
• to leave something as it is after putting it down

So while the grammar contains a state + にする pattern, the natural meaning is simply:

• leave the glasses on the desk

It separates the two linked parts of the sentence and makes the flow easier to read.

• 眼鏡を机の上に置きっぱなしにして、
• 出かけてしまいました。

The comma is especially common when:

• the first clause is a bit long
• the speaker wants a small pause
• the sentence describes a sequence leading to a result

It is not absolutely required in all writing, but here it is very natural.

Not exactly accidentally left home in the physical sense. It means more like:

• I went out
• I ended up going out
• I went out and now regret it because of what happened

The regret is tied to the situation: the speaker went out with the glasses left behind.

So it is better understood as:

• I went out without my glasses, unfortunately rather than
• I accidentally walked out the door

You could make a sentence like that, but the nuance changes.

Your original sentence focuses on the sequence:

1. the glasses were left on the desk
2. then the speaker went out

If you say:

• 出かけてしまって、眼鏡を机の上に置きっぱなしでした。

it feels more like:

• I went out, and it turned out my glasses had been left on the desk.

That version sounds more like a later realization.

So the original order is better if you want to emphasize the action chain that caused the problem.

A good breakdown is:

• 眼鏡を = glasses + object marker
• 机の上に = on top of the desk
• 置きっぱなしにして = leaving them there / leaving them sitting there
• 出かけてしまいました = went out, unfortunately / ended up going out

So the whole structure is:

[object] を [place] に [state-expression] にして, [main verb] しまいました

A natural English gloss would be:

• I left my glasses on the desk and went out.
• with the added nuance:
• I carelessly left my glasses on the desk and went out.
• I ended up going out without taking my glasses.